QTR-Quadtone RIP

I am making a curve for carbon sepia piezotones on an Epson 4000.  
When printing the ink separation test page, the tutorial states to 
find where the inks max out.  Is it normal for the 4 inks to max out 
at different percentages?  The portfolio black maxes out around 65% 
but the other three inks continue to darken until around 80% when they 
finally level off.  Which number should I use?

Thanks,

Jenny