Hi Niko, for your case it would be a process like this:

To start with take that large side-wall surface, and untrim it to make it a full sheet. that looks like this:



Then use Construct / Offset to generate an offset surface, like this:



Now trim your object with the offset surface, here you can see where the intersection curves are generated:



Then delete the offset surface:




Generally that would be the method to make sure you are trimming off a constant distance strip.

An isocurve is not guaranteed to be a constant distance away from an edge curve unless the surface is very regularly shaped. Isocurves come from the UV parameter space of a surface and will generally follow the shaping of the surface's control points. If control points are bunched together, the the isocurve is affected by that.

Here I'll show an exaggerated surface to demonstrate:



You can see that the isocurves extracted from this surface are not a constant distance away from the surface's edge - they narrow together where the surface narrows together and expand apart from one another where the surface's control points expand apart from one another.

But isocurve trimming can be good for things where you know the surface has been generated through profiles along a path like a sweep or a fillet surface.

For constant distance type things you may be better off using offset or Fillet which incorporates offsetting within it. That will make sure you actually get a constant distance which you otherwise won't be guaranteed with isocurves.

- Michael