VSS With Trajpar

Creo 2, M120

I have a VSS where I need the value of one radius to vary along the sweep from a large vaue (X) at the start to a much smaller one (Y) at half way back to (X) at the end.  Can I do this with a relation and trajpar?  If needed, I could split it into 2 sweeps where one went from X to Y and the next from Y to X.

Thanks,