Calculating Flow Rate

Hey guys! I had a question. When calculating flow rate for the tank I do the simple equation of powerheads + sump pump/ gallons of water = flow in system... Now.. I was wondering, since I just set up my first sump, do I use the entire water volume of the entire system or do I just use the DT water volume for that equation? I mean, it would make sense to use just the DT volume because that's where the flow is but I was jw to be sure. I have a 100gal DT with a 55gal sump that has about 40gals of water in it when running. So if I use just the DT, I have 3 hydor 240gph, and a hydor 600 gph, plus the setting I have my sump pump at is 700gph. So that would be ((240x3)+600+700)/100= 20.2x flow. But if I use the total volume of 140 it's only 14.4x... So I am wondering which to use because that's a big difference and I'd like to keep the flow over 20x. I could go higher with my sump pump which goes up to about 1500gph so that wouldnt be a problem. Just let me know what you guys think. Thanks!