Maybe this was my issue:
I’ll need to try ‘ribbon offset’ and see if that was it…
It’s weird I can’t find any threads on this:
ahh I had the wrong nomenclature:
‘draft curves’
this is so cool, I’ll remember the command eventually 
Ahh I need to remember ‘draft angle analysis’ too 
Fun tool 
Maybe there’s an easier way to do this, but I basically used draft analysis on the subd, and locked it’s layer with transparent rendering, then manually traced the mesh with poly line on mesh.
Then I’ll try using that to further simplify the mesh that’s used for calculations, and try ribbon offset again to trim it o’ course.
yeah just extruding it straight again or tapered might be the quickest
yeah basically using the previous workflow I did further above
just doing a more precise cut around the object of interest I think
some rebuilding smoothing extending etc.
that’s the best way to split imo:
The first time I ever did something like this I was a total Rhino newbie and spent two weeks setting it up (on a much more complicated part) – without even knowing it would work until the setup was done.
That’s how I knew this would work. 
Now let’s see the new results:
more accurate version:
