Sounds like an exciting project!
I'm running a Hammerhead on my system, and it's supplying ~1800gph with my current design - supplying the DT and a LifeReef VS-36. Several months ago, I also had a 75gal and a LifeReef VS-24 on the same supply pump. At that time, I didn't know what my total head losses were, but I can tell you it didn't have enough power to run both skimmers. I have a Reeflo Marlin that I will experiment with in the future to see if it can run both skimmers and supply enough to the DT and 75gal. Long term, I'm going to purchase a skimmer big enough to handle my entire water volume (500gal current w/ future goals of 800gal).
To determine my actual head losses, I use a kill-a-watt meter to see my power draw and then line that up with the corresponding head loss and gpm values. I found this thread helpful (
http://www.reefcentral.com/forums/showthread.php?t=1470703) and created my own excel file with the data. I can send that to you if interested.
Speaking from recent experience of upgrading to a larger system, (
http://www.reefcentral.com/forums/showthread.php?t=1831347) I recommend having the capacity to perform a 20-25% water change at all times. You'll see in my thread that my current design allows me to easily change 10% of my total volume with just turning a single valve. I plan to upgrade to allow 25% in the future, because 10% just isn't enough if I'm trying to raise or lower a certain water parameter.
Do you have plans for a QT system? You'll find a 55 gal QT tank and a smaller 20 or 30 gal hospital tank very helpful as you aquire livestock.
I look forward to following along!