I use a calculator. For each range of paper grade there will be a deta-exposure that produces an image that is 'just a little brigher' or 'just a little darker' to my eye. So in my case I have tested to find that at the low contrast end the multiplier is 1.4 and at the high contrast range it is 1.1, and around 1.3 for the mid range.
Likewise if a f-stop timer is used, the 'stop-value' needed for a given degree of change will be different depending on end of the contrast range you are using, so an f-stop timer its not necessarily a panacea. Ideally I would like to have a "D-stop" timer. DENSITY-stop, which would be programmed to produce a uniform density change per increment, depending on which contrast grade I am using.