Great question, but need more information, OR you already have all you need to know.
You say you get a
dynamic (flowing) pressure of 30 psi at 10 gpm, at the POC. All you need to do is calculate the pressure loss for the back flow assembly, the friction loss of the pipes, and the loss of the valves you are using. Calculate that to the furthest head, and you can design your system. You may have already done this, when you say the PSI loss to the furthes head is 9 PSI. Sounds like a lot, and can be reduced by upsizing the PVC diameter, and chnging the type and/or size of valve. If you have 21 PSI at the furthest head, you might consider MP rotors, or maxipaw impacts. Both work at lower pressures, and the MP rotors work on lower volumes, meaning you can put more per zone, but have longer run times.To get good coverage patterns, most pop up sprayers need about 30 psi. Rotors even more.
Friction loss in schedule 40 PVC is of course dependent on pipe diameter. For each 100 feet, my charts tell me:
3/4 inch loses 7.74 psi/ 100 feet. (also, this size should not handle more than 8 GPM due to water velocity) 1 inch = 2.39 pSI losss/100 ft 1 1/4 loss is 0.63, 11 1/2 loss is 0.31, and 2 inch is 0.09 loss.
So, upsizing long pipe runs will increase pressure at the head. Valves also eat pressure. I use Weathermatic silver bullets (loss 3 psi at 10 GPM) Many of common Rainbird valves are twice that.
Now, a very important distinction. Dynamic vrs static pressure. When you say you measured 30 psi at a flow of 10 GPM, I am assuming you took the pressure while the water was flowing at 10 gpm. I know you probably did, but other reading this might not realize that if you take the pressure with no water flowing, that is static pressure. If one were to measure the pressure, then turn on the water and measure the flow, that would be different. Dynamic pressure is meassured at a certain flow rate. Usually a special device is needed to accomplich this, and few home owners have one. ( I don't have one, but want one....hint to wife for Christmas
)
If the 10 GPM @ 30 psi is not your dynamic flow, write back for more explaination. Also, how did you get the 9 PSI loss at the furthest head? Sounds to me you know what you are doing.
Hope I helped, at least some. Jeff