Could you possible create a sketch in isometric or show what you want in side, front and/or top views? Having difficulty understand what you want to do.
Maybe this displays better what I'm trying to do:
The flat part stays untouched.. the rest of the "block" tapers on the inner dia. to minus 0.5mm and the outer dia. to minus 1mm. But I'll have to taper it 2-ways, but I'm not sure how to.
Here is what i achieved (not sure if this is what you seek)
I used a helical sweep for the cut (but i could not achieve the straight line transition edge between the FLAT and the tapered area)
I think making it planar will be quite difficult (rather impossible)...as the plane only needs 3 points for definition and the one given in the image need 4 points so i guess it might be a warped (sorry 4 point patch) surface.