I have a surfacing situation that I've not been able to overcome.  In a piece of tooling, there is a sculpted contour that has 3 small undercut areas.  The goal is to remove material such that the undercut surface become purely vertical.  The undercut starts in the middle of a surface, so initial efforts at creating surfaces/cuts bases on vertexes and lines in the model have not been successful.  I've thought about a 3D sketched line to sweep along a path, but am unable to create the proper path as I'm not aware of any method to find the tangency of the surfaces where the undercut begins.  Does anyone have any suggestions?  (SW2015 file attached, also have SW2016)

Thanks in advance for the suggestions!

Scott