Let's back up a little...
Jeff hit the nail on the head above. The max pressure you should be exposing your system to by plumbing code is 80 psi. Best if you don't expose clear housings to over 60 psi. So you need to reduce the pressure reaching the system, rather than worry about measuring the high pressure accurately. You need a pressure reducing valve:
Recommend you
DO NOT run the system with the current high pressure - you're playing with fire...
Russ