You can sketch the spline only specification and post here.... Splines are not a national security secret.
I think ~ you're trying to measure the position of each individual spline feature of size relative to the datum’s.
There's not a conversion formulae. You would establish your 3D zero from the datum’s and then measure the derived center plane of each spline feature. Measure at many opposed surface locations of each spline to establish the as-built center plane of the spline and compare the measurements to a symmetrical tolerance boundary oriented and located relative to the datum zero.