I'm "double-calibrating":
I have an Opemus 6a with colour head, which includes an ND filter along with the common C/M/Y. Great for fine-tuning exposures. But only up to 6x6...

I also have a Durst 138S with condensor head. I use this with grased paper, or hand-held Cokin filters under the lens (!). This seems to give an adequate range with most VC papers. I have test-printed the paper/filter combinations with the step-tablet in the negative carrier; without, with yellow, and with magenta filter. All at the same time and aperture. That way I know exactly how the filters influence the exposure. Exposing to desired contrast grade is done by simple (mental) arithmetic... It's a kind of split-grading, except that I use unfiltered and either yellow or magenta filtration. So far, it seems to work - to my considerable surprise!