Dry-down timer or dry-down dimmer?
If yours is like mine, it has an intensity knob (letters a, b, c, . . .) and a drydown knob (in percentages).
If you are asking about the operation of the drydown feature, that is a matter of testing with whatever fiber paper you are using. It is worth your time to establish the drydown compensation of your paper if it is something you use regularly.
I use so many different papers, basically just dial in 6%, that seems to be ballpark, maybe 5% if the print is going to sepia and 7% for gold and selenium.