QTR-Quadtone RIP

I was excited to try out the split tone features in the new version.
I'm running os x 10.4.2 on a ppc, with an epson 1280 and UT2 inks. 

The sliders started at all 0's. When I tried to get, say, highlights
to be 100% curve 1, 0% curves 2 and 3, I could not set that: I got
100% curve 1 and -100% curve 2 or -100 curve 3. It seems that it's
trying to get the sum of the three curves to 0. Or something. So I
left them at {100,-100,0} (and similarly for the midtones and
shadows...) and my print was just wet with ink - completely black, and
running off the page. Clearly this wasn't right.

I eventually saved out this setup as a preset and edited the
custompresets.plist file, setting the -100's to 0's. Doing this, I
could print out fine. And what's more, I could adjust this initial
setup to get any combination I wanted. It seems that once it started
with the sum of the three sliders at 100%, any adjustments I made kept
this same constraint.

This sounds like I've got a solution. Have other people experienced
this, and am I missing something? And does anyone familiar with this
sense that this "fix" is in fact a proper one?

Thanks, and thanks to Roy for this wonderful thing.