SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

For the detailed explination, do your research at www.irrigationtutorials.com. The author has exactly what you're looking for there.

But here's the short-hand preview of what you do with psi.

#1 Determine how much PSI at the spray head you need for your design. For example, most spray heads will indicate something like a 15' throw spray pattern at 35psi. So if you want to space your heads 15' apart for head-to-head coverage, you've got to have 35psi at the spray head.

#2 Determine your starting pressure. You've already done that. You know the static pressure is 65psi. For for this theoretical example, you can lose 30psi through the system and the spray-head work as designed.

#3 Add up everything that is going to rob you of pressure. In some cases you'll need to find specifications for your equipment. For pressure losses due to what is called "frictionall losses" in the pipes, you'll need some friction loss tables such as these: http://www.hunterindustries.com/Resources/PDFs/Technical/Domestic/LIT091w.pdf. Most places you're loosing pressure are dependant on the gpm, USUALLY the higher the gpm, the higher the losses.
Here's a breif list of where you will lose pressure (of course depending upon your setup, you may or may not have some of these):
Water Meter
Control Valve
Backflow preventer
friction losses in pipes

This is where designing pipe sizes come into play. For example, if you are using 3/4 pipe with 10gpm over long runs, you could easily loose more than that 30psi given for the example. If so, then you'd have to upside the pipe to reduce the pressure losses. Here's a simple example based on the charts:
Lets say you have a 100' run of Sch40 PVC pipe from your water meter to the control valves and you plan to run at 10gpm. If you use 3/4" pipe, you will loose 7.77psi of pressure in just that pipe (not including the backflow preventer that should be between the meter and the valves). If you upside to 1" pipe, you'll only loose 3.75psi in that pipe. If you upsize to 1-1/4" pipe, you'll loose less than 1psi in that run of 100' pipe.

#4 If your starting pressure minus all of your losses is at least equal or more that what you designed for at the heads, then your golden and can start turning some sod. If the starting pressure minus all of your losses is less than what you designed for, the spray heads will not throw the water as far as you have designed for. You'll have to go back and change the design to either reduce gpm by breaking the system up into more zones, use bigger pipe to reduce friction losses, and/or change the desing (such as spacing heads closer together so that your required working pressure at the heads is reduced... of course this will increase the number of heads, likely increasing your gpm, and therefore likely requiring you to break the design into more zones).