OK, I've received some questions about the overflow holes and the design of the overflow boxes. I spent quite a bit of time researching this and did a bunch of calculations, some of which may be wrong so please check my math.
Let's start with bulkhead size, since everything else is sized off of the holes in the tank. I plan to flow ~1,000 gallons per hour through my sump (about the middle of the 3-5 times tank volume many recommend). Gallons, per hour isn't a good way to size a bulkhead so I did some converting:
(1,000 gal/hr) * (231 in^3/gal) * (1 hr/3600 sec) = 64.1 in^3/sec.
So, I'm flowing 64 cubic inches of water per second. Calculating the area of the holes, allows me to determine how fast the water will be flowing in the overflow. I don't know the inside diameter of the bulkhead, but it should be close to the outside diameter of the pipe. A 1-1/2" PVC pipe has a nominal outside diameter of 1.900". The area of that pipe would be:
(1.900"/2)^2 * 3.14 = 2.83 in^2
Based on everything I read, you really want the waterline to be approximately half way up the bulkheads. This will allow the two boxes to act as a single body (no water height difference between them). So, we can use the .864 dimension for 2 pipes filled halfway.
So, now we can calculate how quickly the water will be moving. With 2 holes:
(64.1 in^3/sec) / (1/2.83 in^2) = 22.65 in/sec
To me, this is much easier to visualize than gallons per hour or some other complicated measure. Just lay a ruler down and imagine water going 22" in a second.
With 4 holes, it would be 11.3 in/sec.
With 6 holes, it works out to 7.5 in/sec.
I consider these to be minimum velocities, since I don't actually know the inner diameter of the bulkhead. I had to goals when picking the number of holes. First, I wanted it to be silent. That means slow flow. However, if the water flows too slow, the detritus sinks and could build up in the overflow. I'm not a fan of extra tank maintenance so I wanted to avoid this. To my completely untrained visual image, I imagine that 7.5 in/sec is just fast enough to keep the detritus suspended without being to noisy.
You can do similar calculations with different bulkhead sizes. Using a smaller bulkhead would require more holes to achieve the same flow at the same water velocity. You could use bigger bulkheads, but that requires bigger holes. For safety, you need to keep holes away from the edges, so that means bigger holes need to be drilled lower. This affect box height and how far the water falls into the inner box as discussed below. Also remember that the bulkheads are holding the boxes in place (especially the outer box), so more bulkheads may be better. You can space the through holes however you want. I chose not to put any right in the center, since that is where my siphon pipe will be. I didn't want the flow to all go through the center hole and end up with deadspots in my overflow box, again to avoid accumulating detritus.
Let's move to the inner box now. My box will be 42"x3" and 5.5" tall. The size of this box was driven by my choice of bulkheads. According to the BRS website, a 1-1/2" slip/slip bulkhead is 2.87" long (dimension B on their picture), has a diameter of 3.38" (dimension A), and requires a 60mm hole. The box is 2.75" on the inside (3" deep minus 1/4" for acrylic thickness), which is slightly less than the bulkhead, but I will be able to angle it into place.
The height of the inner box depends on two things - the bulkhead size and the distance to the top of the tank. We'll start with the the bulkhead size. Since the bulkhead has a diameter of 3.38", I need at least 1.69" (3.38/2) from the center of the hole to the bottom of the box. I didn't want to push it, so I made this dimension 2.5" (don't forget you lose 1/4" for acrylic thickness). You could install the bulkhead backwards and make this dimension smaller, but then you have to tighten the nut inside the little box. Since I have big hands, I chose save myself some frustration and didn't go that route.
The top half of the inner box is driven by your tank height and desired water level. Because my tank has a rim the calculations were a little more involved. The box need to have at least 1.69" above the hole centerline to cover the bulkhead diameter. Again, I can round this to 2.5" to give myself a little room. However, that would mean that the through holes would be less than one diameter from the edge of the tank. To be safe, the centerline of the hole should be 1.5 diameters from the edge. This ensures that the top edge of the hole is at least one hole diameter from the glass edge. A 60mm hole is ~2.36". 2.36*1.5 = 3.54" So, I had my holes drilled 3.5" from the top of the tank. My box will have a lid and has a rim which is slightly lower than the top of the glass, so I made the box 3" tall above the hole centerline. Thus, the inner box is 2.5" + 3" = 5.5" tall.
Waterline in the tank is determined by your weir height. My box won't have teeth, but that doesn't really affect the calculations for water height much. My rim extends down about 1-1/4" when measured inside the tank. Subtracting 1/2" for the box lid and a little gap, leaves 3/4". This is the height I selected for the bottom of the slot in my box. The actual waterline should be ~1/4" above this (your number will vary based on length of overflow, return volume, etc. but I think 1/4" is a reasonable number for most) but cannot be lower than this. Setting the bottom of the slot approximately even with the rim will allow for some water rise during start-up or other off-nominal situations. It also minimizes the distance water falls into the inner box. In my case, the distance can be calculated as follows:
3.5" (box waterline to top of tank) - 1-1/4" (approximate distance from top of glass to tank waterline) = 2.25". The only way to reduce this height is to use smaller bulkheads, but you would have to go really small to make a significant difference. 1" bulkheads require a 45mm hole per BRS, so that would reduce the number by approximately 0.6".
OK, so if you're still following along with this math lesson we have a box height of 5.5", depth of 3", holes 3.5" from the top of the tank, and the waterline set slightly above the bottom of the trim. Let's look at length. I couldn't do a full coast to coast because I'm planning to run Sea Swirls in the back corners. So, I went with a 42" overflow. The tank is 72" long so that will leave about 15" on each end for the Sea Swirls. It also means that the overflow is closer to the back corners than it is to the front of the tank (about 27"). I hope this will prevent any deadspots from forming in the back corners. Since the Sea Swirls will be pushing water towards the front of the tank, I don't think I'll have any deadspots there.
Now that the inner box is sized, let's move to the outer box. First, I wanted the outer box to be positioned lower than the inner box. This would ensure that floods occur in the fish room and not in my basement. Second, there really is no limit to the sizes on the outer box in my case. However, making it too big would stress the bulkheads and I would have to add some support. I chose to make this box 6" deep and 7" tall. That should give me plenty of room for plumbing without adding too much stress to the bulkheads. The outermost holes in my tank are 29" center to center. Add 3.38" for the bulkheads and you get 32.38". So, I made this box 36" to give myself some room (again don't forget to subtract for acrylic thickness).
So that's it. Six 60mm holes 3.5" from the top of the glass, a 42"x3"x5.5" inner box and a 36"x6"x7" outer box. If any of my math is wrong or you have questions, please let me know. I hope somebody finds this helpful. It took a while for me to research everything and figure out how I wanted the holes in my tank. It's one of those things where you only get one chance at it, unless you want to drill it yourself.
In case you are wondering, my overflow boxes are being made by David Bombara at Acrylic Creations. He's somewhat local to me and seems to have good reviews. He's been awesome to work with so far. He called me the other day just to discuss what I was doing and suggested a couple of minor improvements. He didn't just make what I ordered and let me deal with it when it was wrong, and I really appreciate that.
-Ivan
