Paul Butzi has an article about running some fairly simple
tests to produce the filter settings for variable contrast printing.

When done, changing the ratio of yellow/magenta changes the
contrast but not the overall exposure time. Have not done this
myself yet but need to. Right now I split filter every print,
even the 'easy' ones... this would make it easier for me...

http://www.butzi.net/articles/vcce.htm