Making an alignment to a contoured surface that’s the primary datum.

This ring is almost completely model based with very little dimensions. There are no targets. Just the big contoured surface that is Datum A. Then a rib on the left is B and rib in the front is C. I’ve been struggling to get good results and the way I’m currently doing it. The part does have a fixture that was made for me to use while inspecting the part which mimics the contour of the part and keeps it stable.

Right now I’m measuring a bunch of points all over the surface and also points on the B and C datum’s. Then I 3D best fit all the A points. Then recall that alignment and 2d best fit rotate only all the B and C points. I repeat this process three times before defining my main alignment. Does this seem right or and I’m way off the mark here? Any help is appreciated!