Off center sheet metal follow bend curve.

Hello,

I have a problem creating a bent curve on a sheet metal.

This is the situation, I need to create a curved bend following a circular frame.

The sheet metal rests on the frame (not centered) and follows along for some x cm (here 15) so it can be welded later.
What you see below is just an extrude of a sketch I created by a 'convert entity' of the frame's curve.

But this way (although it looks good) I loose the unfold feature of course, so that's not good...

So what I need is to create a curve on a sheet metal by using the 'sketched bend' feature right?
2 times, to be exact. Once for the angle from the sheet to the frame, and a second on the other side to follow the frame diameter.

Like this:

But how to calculate the initial 1st curve?

How to calculate that curve's exact tangent degree value?

SolidworksSheet Metal