BBQ Update

I was thinking it would be fun to make the PB BBQ have a value gradient across the quilt. I like doing that in quilts. This is a rough EQ6 sketch. Obviously the 'threshold' between light and dark is where my problem might arise. I don't want it to look muddied. I want it to flow evenly from one region to the other. Also, I was trying to figure out what to do with all my beautiful medium value fabrics that I had selected, that didn't really fit in with the previously posted design of strictly lights/darks.

Surely there must exist an algorithm to tell me the proper value for each piece given a desired outcome. I could scan all the fabrics I want to use in the quilt and the algorithm software could rank the values of the fabrics, assigning a number, and I could choose from the values and place them in the design accordingly (like paint by number). Then again, I guess with practice and experience, my brain is like one big algorithm.