Hm, i'm not sure that the method is reliable.
I edited the surface such that it is simpler, now it's degree 2x1 and single patch on both u anv.
It is in other words linear in one direction. ( The isocline should be "straight as a line")
Then i drew a sketch on the plane and in that i drew 2 lines to use as vector definitions for 2 Isocline curves. The two isoclines are totally different and none of them intersects the minimum distance point. See attached 7.5 model.
Regards,
Tomas