Splitting splines is awkward. I only managed to do it when I started from a spline split in two in the middle, then went from there, trimming the ends. If the spline already had offset constraints, SW refused to split it. I wonder if there's a ton of references to the spline at the time you realized you need to split it. Is there a better way other than redoing from scratch? On a side note, is there a way to add an offset dimension constraint after the construction? I couldn't find a way to reflect it in the sketch. I know it is there because when I try to move the point, it rebuilds the spline Finally, making the sketch fully defined without using "Fix" constraints is also non-trivial. I'd love to see how it can be done in a pro way ;-) Thank you for the teaser; it looked way easier than it turned out to be.

@Jesse Davis second try using surfaces is cleaner and does not suffer from issues with spline offsets. But technically I needed a second side plane sketch, though a pretty simple one. I wonder, though, why SW is so picky about a particular approach. Spline offset is problematic, but surface offset with the original surface made from the same spline is totally fine... it feels arbitrary in the meaning that some approaches are just supported in SW better than others.