Keith,
I don't know if this will help, but this is from my 617.95 manual.

To check boost pressure, hook a tester up to the switchover valve with a "Y" connector, and drive the car in "L" or "S". Very briefly, increase the RPM's to 4000. I believe you will need to ride the brakes. You should also have the tester in the car with you(long hose fed through the firewall?) or have one that remembers the reading.

The boost pressure should be: 0.7-0.8 bar gauge pressure

To all: please expound on these directions if necessary.