The one I built is adjustable in .01 increments, but I have a knob that adjusts them by .05, or 1/20th of a stop. I have yet to find that I need more precision than that.
My color head uses density units, 30 per stop. I find that .05 is usually sufficient there for color, but not for black and white. So keep in mind that there may be more need for fine steps if you are doing black and white vs color. But either way 1/30th of a stop is more than enough.
I tried a metronome after hearing such great things bout them from others. I realized it just won't work for me since I lose track of the time in the middle of any long exposure (I can't seem to count and focus on the dodging or burning at the same time). To keep both hands free I use a foot switch. I suspect the metronomes work great for people with musical abilities, of which I have none.