In my case I've used mine to objectively get me into the ballpark with a film's exposure and processing variables. Once I have that objective baseline in hand, final adjustments are then made subjectively using my eyes and judgement.
But a more valuable use of the tool for me is ongoing process control after I've settled on a calibration. Every so often when I'm out with the camera I will look for examples of lighter and darker continuous tone objects. Whiteish fences in sun, blackish walls in shade. That sort of thing. I then set the lens to infinity, move close to fill the frame, and make quick highlight (development time) and shadow (film speed) test exposures.
It usually only takes a minute or two and a couple of frames. If necessary I could do the same for expansion and contraction tests, but I rarely do. I'm usually just looking for a quick sanity check.
After development all it takes is a quick check under the densitometer to confirm that the calibration is still valid. Reasonably close density numbers are perfectly acceptable, as I'm not running an analytical lab - just a home darkroom.