Hey @RAllan @MJB-online @Elton_Williams @msol
This is exactly what these nodes are doing. You can find more about their use on this tread: Edit crop region
It’s not included in my workflow / blogpost, but what I would suggest is to get the curves, do some list management magic to translate the bottom curve, create a new polycurve from its intersections and the existing curves and then feed the exploded polycurve to the “…Set Curves”
[edit] now I see that you’re on that referred tread too already 
Cheers,
Dieter