QTR-Quadtone RIP

hi everybody,
i have a problem that i can't solve because i can't see where i do wrong.
i followed the user guide prepared by Tom Moore but when i print the
100stepWedge.tif file i figure out that from 99% and 100% there 
is an evident step. I have tryied to modify Highlight, shadow and
gamma parameter but without success. 
It's not my first time in curve creation and i'm weasting a lot of
paper and ink and it's frustrating.
can anyone help me ? i can post scanned file... evrything could be
helpfull.

thanks in advance
Paolo