Its been a long time since I ran comparator but, the face of the view screen with the measurement rings on it should rotate so that you can align the cross hair on the surface you want to be perpendicular to. Then you should be able to measure "up" the other leg of the cross hair for distance or perpendicularity.
I hope this helps.
Scott
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The early bird gets the worm, but what about the early worm? A.E. Neuman