I think this might work....
1) do a trim check on the part
2) Flip the yellow/grey vectors because you are measuring the gage, not the part.
3) Swap the yellow and gray vectors so that the yellow is on top and the grey is on the side.
4) Edit the trim measurement and set the material to the gap check value and set the depth to the distance away from the part you want the flush check.

This makes sense...give it a shot.