if you are referring to my 108, i searched the net trying to find a calculation that wouldn't involve measuring vacuum and such..i know it's ballpark for sure, but i got that suggestion from inyopools faq i believe it was that said for a "simple" TH calculation, add the length of all your returns to get the TH. then i went to the "curve"...maybe garbage in, garbage out at least as far as the TH, but i don't know how to derive it any other way that i "get".

a summary and more information if it helps includes:

15,600 gallon pool (u.s.)
a skimmer and a main drain.
1.5 inch piping everywhere.
1 hp sta-rite max-e-pro pump which i believe is supposed to develop in excess of 60gpm at TH of 50.
sta-rite sand filter max rated 62gpm
vacuum that requires a 1hp or 1600 gallons per minute flow, i believe it is.
4 -90 degree bends, 3-45 degree curves in the pump shed. (don't know how it was bent underground as the pool already existed)