draining calculator

[Invigor]

i found a simulator sort of calculator on reefs.org that puts all the specs of a pump against the friction co-efficients of pipes and fittings and such..but does anyone know where I can find or how to do a calculation for my overflows to see the max amount they can drain, given the size of pipe/fittings used to return to the sump? (i'll be drilling 2 holes, to fit either a 1.5" or a 2" bulkhead)

I'm modifying my tank to handle about 2500-3000gph, the returns are going to be 1.5" but i'm debating on using 2" for the overflows. dunno why i don't just go with 2" pipe on the overflows and have it done with..

I plan on using flexible tubing for it all, and as little rigid tubing as possible.