The variable contrast filter set I use goes from a grade of -1 (very soft) to 5+ (very hard) in 1/2 step increments. The exposures stay the same for all filters from -1 to 3 1/2 and need to be doubled for the range from 4 to 5+.
The question is what does the term "exposure" refer to; the exposure for the darks, the mid-tones or the highlights? With my filter set the key exposure, the one that stays the same, is the one for the highlights.
I do test strips until I get the highlights right. That determines the number of seconds set on the enlarger timer. If the shadows are too grey I need to select a higher contrast filter; if too black a lower grade filter.
The alternative, pegging exposure for the mid tones is too hard because changing the contrast grade shifts two things simultaneously; the highlights and the shadows! I find it less frustrating to have one thing that stays the same while the other thing is changed up or down.