QTR-Quadtone RIP

I've nearly manually completed a curve with MPS K7, all seems  to go 
all right till the 90%, the end I can't get a reasonable result i.e. 
density and linearity together.
Checking the existing curves, the overlap is obvious, what is less is 
the overprint and I think this is the way the dark patches are built.
My question is whether there's a way to predict overprint values or 
it's just a try&see process.
Thanks.
Olivier