David, Keith:
Thanks very much for the feedback. David's solution using Terrain Specification - Building Pad worked.
Foundation and Retaining Walls appear relative to the building pad reference.
Slabs on the other hand are separately set under Slab Spec to a 'Floor' by default, with options to reference from 'absolute', 'terrain', 'ceiling', 'roof'.
It's confusing why there needs to be two separate reference points, one for walls, and one for slabs. It seems they should be tied together for each slab.
David's solution is available in the Pro version. As I am using the lower end Architectural, I am thinking to try and reset the dwg imported contour elevations from sea level to an absolute zero. I'll circle back here with results.