At a guess, those numbers aren't random :) 0 being the near point (black) and 20cm being the far point (white) seems like reasonable values for a small scale scene. If you lower the Max Z, more of the watch will get brighter (the far point for pure white gets closer, things don't need to be so far away to reach full white).
Those numbers always depend on the scale of the scene - a watch, a living room, a street, will all need different min and max values based on their scale. Out of interest, what were those values when it wasn't working?
Thanks!