Ok, to start with, "die cut" is the translation Google offers for what we call "corte troquelado" in Spanish so it might not be the right naming convention for this kind of cut. I would appreciate if you tell me what´s used in English.

We need sometimes to make a cut in a sheet metal of a figure we might or might not use in a particular part depending on it´s particular final assembly.  I know "microjoints" is the standard way of naming/doing this in CAM software. Usually these microjoints might be solved by the CAM software but this requires one further step so we would like to model this in the .dxf itself.

If I start with this:

 

and edit the sketch later by using trimming tools I usually end up loosing the ability to size the cut properly.  I would like to keep this figure completely defined so as to be able to change dimensions easily.

After I use power trim this is all "blue"

What is the right way to do this ?