I'm not quite sure what exactly you're trying to measure but if you play with the mat'l thickness parameter (let's say you enter -3)you always can measure points offset from that surface by such amount.(A good example is measuring the checking fixture surface which is usually 3mm offset from cad data.)

Another thing you can do if you're comfortable playing with MDT is you can create(offset,copy,rotate,etc...) cad features based on current cad data.(I myself sometimes draw solid clampings so I can add better clearance move...)

Hope this helps.

Regards,

Ray Xing
Quality Control/CMM programming
Metrican Mfg., Ontario