Terry,
I don't make internegatives, but during this last year I took a quick look at the area and was amazed at how well (at Sandy's suggestion) a simple, straight-line curve worked. I make these curves with just two coordinates: (0,0) and (100,100). Then the ink limit controls the final density.
In the 3MK, your Yellow and MK are the only inks that will have significant UV density, and they will be relatively close to each other. So, I'd start with just those two inks in a straight line and see what happens.
Good luck,
Paul