try to look into the attached file...If I good understand You need find highest ponit of curve over some datum plane.
Try to change problem of highest point to problem of nearest point of another datum plane whichmust beplaced over curve and parallely with first datum plane. After choocing feature from type of distance analysis, itcan create two datum points, oneat datum plane and secondat curve. The second is thatYou are finding.
You can easily solve this problem by using a UDA (User Defined Analysis). Create a field point on the curve you need to measure. Next create the distance measure from the field point to the plane. Now create a group of just these first two features. Immediately follow this group with a UDA feature of the distance measure over "entire domain". You will see that one ofthe results returned is maximum distance (as well as minimum distance). Use this result. Keep in mind that this result is now avialble for sensitivity and optimization analyses.