There is no PR to aim for. Zones should have been designed per available flow and area to be irrigated, with a little overhead for flow and pressure fluctuations. While you can play with different nozzles a bit after install using the adjustment screw, that would only be to fix poor design. The rest of it (ex. PR) comes into factor when scheduling the controller.
Also, don't know what you mean by "75% of the GPM recommended on irrigation tutorials". Assuming all the radii and nozzle pressures are the same, the design GPM per nozzle should be an exact multiple of the output for your 180 degree arc on the other non-180 arcs, with a few exceptions. You will probably never achieve this exactly with PGP reds, but you need to get as close as you can with the nozzles provided without having to use the adjustment screw excessively.