QTR-Quadtone RIP

Hi,

I am attempting to make my first QTR curve and have a question.  I understand 
how to compare a lighter ink's 100% value to the next darker ink to find the 
equivalent density in terms of the darker ink.  Where I lose it is with a 
third lighter ink.  I can find the third lighter ink's equivalent percentage 
compared to the next darker ink, but I don't understand what it means to convert it 
to a density relative to the darkest ink.  Can someone explain how to do this 
conversion or point me to a tutorial that uses three inks as an example?

Tom Wells