How to locate a feature on a different surface to the sketch surface

I have a flat plate drawn in DS Mechanical. On one side of the plate is a round pillar, projecting out from the plate. On the reverse side of the plate (i.e., the side without the pillar), I want to create a round depression co-axial with the pillar. So I need to locate the centre of the pillar, on the reverse side of the plate. I have played with the 'project' tool but it does not seem to have any functionality that helps in this. How do I do it?