I don't use an f/stop timer; what I do is count beats from a metronome and time my exposure in beats using 1/4 stop increments. I set the metronome at 72 beats/second so its slightly faster than a second for each beat.

It all accomplishes the same thing as using a dedicated timer. Using the metronome lets me concentrate on dodging/burning without having to take my eyes off the print to read the timer.