If we have this mapped to a variable-member in a record/rep-spec can we simply assign to it or do we have to do additional mathematics to get the delta of the LSB's to be correct?

If we have this mapped to a variable-member in a record/rep-spec can we simply assign to it or do we have to do additional mathematics to get the delta of the LSB's to be correct?

The generated C has exactly the same semantics as the original Ada source code. You don't have to do any manual fixups.