The easiest way is to assume 45-65 feet of head.

The most accurate way is to measured it:

Total head = Return Head + Suction Head

Return Head = 2.31 * Filter PSI
Suction Head = 1.13 * Pump Vacuum (inches mercury)

If you do not want to purchase a vacuum guage then you can assume that suction head is about 40% of the return head (only works for no solar or extra return features) so

Total head ~ 3.2 * Filter PSI

The hardest way and probably least accurate is to use the plumbing configuration. Problem with this is not all PVC pipe is the same friction coeficients and it is difficult to find friction factors for every plumbing element.