This is only a partial solution, using the ‘Z’ (normal) of Plane Fit instead of ‘World Z’. The problem is that Plane Fit is “upside down” at certain angles.
InternalCross_2019Jul7a.gh (13.3 KB)
This version is more robust. Instead of Plane Fit it uses the first two segments of the curve with LnLn (Line + Line) to create the reference plane and Z-normal:
InternalCross_2019Jul7b.gh (14.7 KB)