Parallel Blend with boundry from isocurve (file attached)

I made this work before but had to rescale, and now it doestnt align when I use blend edge again.
Tried to claculate a decent manual blend by extracting the top border curve of a copy that I edge blended(Green) and an extracted isocurve (0.020 in red) but making the cross section curve blends is proving unreliable for such a surface.

Is there an easier way of doing this?

As long as I have a consistant border (Red) it does not matter terribly how it is positioned or how the blend comes out.

Thank you all. I hope this is reasonably clear.
Parallel Blend.3dm (547.1 KB)

Hi Simon - I think the whole thing would be a lot easier if your perimeter shapes were cleaner G2 surfaces all around rather than built up from a series of g0,g1, g2, surfaces-


Thank you Pascal.
I rebuilt it with a sweep 2, but this doesnt help with skewed isocurves when making my blend sweep2.
Do you have any tips on how to do this, ie: is it my initial curves thats the problem? (projected) or do I need to covert the surface?
Thank you.