Set your Z Step to your finished Z Depth (Desired and Actual) and set # Passes to 1. Then use multiple contour operations, each leaving less stock (Stock+/-) on the contour. You can do this automatically with the Offset Contour Plug-In at:
Plug-Ins -> HSM -> Offset Contour
I was just messing around with an OD contour myself, doing pretty much what you are talking about, so I attached it. Hope it helps.