The surface is non-planar. I've created the profile on a 3D sketch.

I tyring to use cut extrude to create and indent on this surface. 

The cut must be uniformed at 1.5mm.

When I set to:

I get thinning over the top flat surface which is not my intended result I'm after - I also want it to cut 1.5mm down from the flat surface:

and also a cut outside the curved surface on the other side:

How can I perform this cut?

I also intend to use the split feature later on as I want to reuse this cut surface later on as the lid for another part.

What's the best way to do this?