That's too much of an open-ended question. It's like asking "how long is a piece of string".
You need to figure out how low in the display tank your lowest return-point is. If you have sea-swirls or lok-line for example, they could make your lowest point lower than the top of your overflow. Once you've done that, you can work out the volume of water that will dump from the main tank (length of tank) x (width of tank) x (difference between high-water-point and low-water-point).
To that, you need to add the drainage from any overflow(s) you might have. Usually the level in the overflow is lower than in the tank (hence the water overflows into the overflow!). So you need to add in the volume of water that will drain from the overflow... (length of overflow) x (width of overflow) x (difference in height from normal overflow height to lowest point in the overflow).
(This is actually double-counting part of the overflow volume since you included it into the tank-size as well, but a little safety margin never hurt).
Then you have to check that the bottom of your overflow is water-tight. I'm assuming here that it is water-tight (teflon paste on an interior screwed bulkhead, or PVC cement on a slip-fit interior bulkhead). If it's not, then the entire overflow will drain (slowly) into the sump, so you need to add the volume of each overflow as well.
Finally you need to add in all the water in your plumbing. Usually I take a guesstimate for this, but you can add up the lengths of piping, figure out the cross-sectional area of the interior of your pipes using (pi) x (radius) x (radius), and multiply them together if you want to...
Ok, so adding all those up gives you a figure for how much water volume will drain. Now you need to work out how much water volume is available. This is completely dependent on your sump, and how any baffles in it are placed. Here's mine:
There are three compartments, and it's a 18" x 18" x 48" sump. Total water volume is therefore 18 x 18 x 48 = 15,552 cubic inches. Sounds a lot... Let's look at what's normally in use:
- The left-hand skimmer compartment is usually full up to the 8" mark, because that's where the baffle is. So just by multiplying out the width x height x length of this compartment we can see we've used up 18" x 8" x 18", or 2592 cubic inches
- The right-hand refugium compartment is usually full up to the 14" level, so here we've used up 18" x 18" x 14", or 4536 cubic inches
- The center (return pump) compartment is normally full up to the 8" level, again because of the baffle, so we've used up 18" x 8" x 12", or 1728 cubic inches.
So altogether, the space left over in my sump is 15,552 - 2592 - 4536 - 1728 cubic inches, or 6696 cubic inches.
Now this is an
over-estimate because (a) I didn't subtract anything for the equipment that's taking up space in the sump, and (b) Even those plastic baffles take up volume. In my case, the only real equipment is the skimmer (the return pump is external), so I'm going to subtract another 10x10x10 cube of space to compensate for the skimmer and baffles, leaving me with ~5700 cubic inches of volume. Note that it's only the volume above the normal-water-level that you need to subtract.
So, then you compare the space you have left in the sump (A) with the volume of water you think will drain down (B). If 'A' is greater than 'B', you're fine. If 'A' is less than 'B' you will probably overflow in a power cut.
In my case, an inch of drained water from the display tank (8' by 2') is 2304 cubic inches. So at a rough guesstimate, I can drain down ~2.5" from the entire tank without overflowing. There's no way I'll be draining that much water, so I'm ok.
I hope you can see that it's always going to be completely specific to your own tank and sump layout. Hopefully the above can help show you how to figure it out 
Simon.
