If I remember correctly, Ron's barrel valves do not bypass fuel to the return line with the shutoff, it merely closes off the supply from the pump. If this is the case and you are taking your reading from the pump side it will not show nozzle pressure. Also you will get a big spike on the gauge when you lift off the throttle.
About the only time I use fuel pressure to reference anything is after the initial tuning is done I will throw a gauge on the nozzle side to get an idea of what pressure to start with on the high speed.
Most of injection tuning is by feel and time slips, you have to have a place to start so you use the basic measurements, barrel valve setting etc etc to get it running.