The 2.5" pipe down the hill is good for gravity feed (minimal pressure loss due to pipe friction.) The GPM if you opened the 2.5" pipe in that location does not matter unless you do that (and would be at 0 PSI.) The GPM will be the GPM of your sprinkler heads (+ any leaks, so don't leak.)

The 200 feet of 3/4" feed to the sprinklers is likely where the problem will be; much more pipe friction (dynamic head) so pressure *when flowing* will be lower or much lower. How much lower will depend on the flow rate of the chosen sprinklers. Larger pipe will reduce the dynamic head at any given flow. There are plenty of irrigation calculators to help with that on the web. Here's one with links to others. http://irrigation.wsu.edu/Content/Calculators/General/Pressure-Loss-With-Outlets.php

I get 28 PSI loss (so 12 PSI remaining of your 40) for 200 feet of 0.75" pipe (beware of bad results on that calculator if you enter 3/4 instead) with 4 outlets, and 2.5 GPM per head is pretty low for a lot of sprinklers, but you haven't given a number.

To get to "1 PSI loss in the pipe" as you've guessed, your total flow would be 1.65 GPM so 0.41 GPM per sprinkler head, which seems - unlikely.

You can sort what size lateral pipe you need by applying that calculator (if the outlets are equally spaced - if not, you'll have to get more complex with that calculatons) to your actual sprinkler head flow requirements.