The KE Jetronic test pressure is temperature dependent from a chart. That said, for the problem that you are troubleshooting, it is more important to look for a serious pressure drop when blipping the throttle than it is to look for the exact correct pressure value.

You will need a tee set up with the correct fittings to connect a fuel pressure gauge to the system. Consult the FSM or a Bosch Fuel Injection manual for specific fuel pressure gauge hook up points.

Good luck,