The shop manual says 25 to 45 psi at 1500 rpm with the engine hot.
Were the readings you observed with the engine hot (up to full operating temperature) or cold?
The negative battery terminal would be the best ground source (but not needed), the engine block the next best (what I would use), the chassis the worst...
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq