The guage that is vacuum only will give you a more resolution. You could hook it into the CCV drain line going into the dipstick or the dipstick tube itself. You will have full manifold vacuum in the crankcase less any leakage because the PCV "MOD" does nothing to regulate vacuum.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)