offset datum targets for Primary datum

[[Ty...]]

I have a part with ~20 datum targets to make up Datum A. These are at various heights and the part is to be constrained at these datum targets. We have a fixture made to support all of the datum targets as well as constrain them. Normally, I would just measure the fixture to get the datum then constrain the part to the fixture and proceed measuring, but I'm getting tripped up with them being at different heights.

Because these targets are at various heights, I think I have to use "offset plane", but I've never used this before and am having trouble finding information about it. What would be the proper way to get this datum?

Edit: Reading the user guide, looks like I can't use offset plane with more than 3 points

[[Ri...]]

The only way I know to do more than 3 points is to measure the points individually, and create theoretical points that use a formula to offset to points.