I got a bunch of Forte multigrade paper on sale at the local shop. Here is how I calibrated my color head to allow me to keep the same middle gray exposure while changing contrast grades with the color head. It involves adding in RED to the Magenta or Yellow filtration as neutral density, so each "Grade" has about the same exposure to print a middle gray. So, for example, 80cc of Yellow gives ISO R (contrast) of about 120. This will eventually become M 32 and Y 112 after the neutral density (32cc RED) is added, so that the printing time matches all the others.

This is not a complete tutorial (the complete version is here: http://www.butzi.net/articles/vcce.htm)

First I needed to find out how many CCs of neutral density on my color head equalled an exposure change of one stop. The Butzi article mentions using an enlarger exposure meter, but I used a standard exposure meter and it worked fine.

I plotted change in neutral density (CCs of combined Cyan, Yellow and Magenta) versus change in exposure meter reading.