Am I crazy? First of all, thank you for all your replies. I really appreciate your knowledge and experience. I just came up with an idea and want to run it by you for your opinion. I'm probably not the first one to do this but I was thinking of making up a small manifold that I would attach to the PVB about a foot or two away. The manifold would allow for 4 attachments. 3 spots would be occupied by pgp rotors with nozzles that would give me my goal of 15 gpm output and the 4th spot would be the pressure gauge. This would allow me to read working pressure while putting out the 15 gpm. All that would be missing would be to account for pressure loss for the valve (planning on Hunter PGV about 1-2 psi loss), and the eventual length of the piping to the zone and the head (1-1/4 sched. 40 or 1-1/4 poly). I could then swap nozzles with lower or higher output to see what the optimum gpm would be to get that 35 - 40 psi working pressure all the while accounting for the additional loss from the valve and piping that are missing. Am I crazy? Your thoughts?
I like the test manifold idea. Just make sure your gauge will drop as the pressure drops. Like I said earlier, the cheaper gauges will stay pegged at the high pressure even though it may have dropped by 20psi and give you a false impression. If you run 15GPM and the gauge doesn't drop something isn't right.
Also, see if 1" poly will meet your requirements for the length of your lateral lines. The pipe is easier to find and the fittings more common (in my neck of the woods anyway). I don't think 1 1/4" will buy you enough PSI to be worthwhile. I kind of complicated my design and had my 1" poly lateral line for each zone tee off into 3/4" poly runs which then feed the sprinkler heads. There is a company that makes a neat 1" and 3/4" poly tee that connects directly to funny pipe. I probably could have use all 1" poly but it would have been overkill since my heads are using such a low GPM. 1 1/4" poly fittings are also more expensive than 1".