QTR-Quadtone RIP

Hi,
   I suspect that Roy or Paul will be the ones most able to answer this question, but comments from anyone would be appreciated.

Is there any general rule, guideline, theory, etc. about how the cross-over points between different gray inks should be distributed through the density range of a profile?  

Naively, I would think that the cross-overs should be distributed at roughly equal intervals.  But, I have noticed that when I make curves for the UT-14 inkset (where each profile uses only 2 or 3 inks) and follow the procedure for setting the Gray_Val parameters described in the "Calibrate.pdf" document, the lighter ink (or inks) are only used quite high up in curve (> 75% image gray).  This is a consequence of the fact that the the lighter UT-14 inks can actually reach quite high densities by themselves.

I've been experimenting a bit and have figured out how to place the cross-overs at different points, which I think might lead to smoother curves.  But, I am wondering whether my intuition that the cross-overs should be more evenly distributed might be wrong.

Thanks for any comments or ideas.

David