QTR-Quadtone RIP

I am still tweaking my carbon sepia curve for QTR and have another 
question.  I am wondering what is the best method for tweaking a 
certain range of the curve when creating a profile.  I'd like to 
darken (very slightly) the 55-75% range of the stepwedge and lighten 
(again, very slightly) the 20-40% range of the stepwedge.  Is there a 
way to specifically target those areas?

Thanks,
Jenny