Shrink Surface and shrink it to it's border 100%

I am having a hard time wrapping my head around surfaces and their perimeters.

I have an input surface form a project that derives from a bigger surface.

I really need the surface only and nothing of it’s origin.
So I try to do the following…

side = Rhino.Geometry.BrepFace.ShrinkDisableSide.ShrinkAllSides
face  = brep.Faces[0]

This results in something I basically understand and am familiar with.

So, if I’d try to do “limit” this to the desired surface I would think this might help…
But actually it does not help as well.
So I am a bit lost and maybe someone might help here?

I found this, which is pretty interesting but does not help me in this case.


The solution I did is the following.
I re-created the surface as a sum-surface, as this keep it’s proper perimeter.
This can be kind of tedious, but worked so far!