The easiest way to test the boost is to tee into the boost signal line at the overboost protection switchover valve on the firewall. Antwhere from the intake to the aneroid on the injection pump should have the same pressure. The pressure should be controlled to 9-10psi. Any lower suspect: plugged line before gauge, bad turbo or stuck open waste gate. Any higher look for a bad signal hose to the wastegate.
The waste gate isn't adjustable except for the ammount it opens. It works against a set spring pressure.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician