You really want an f/stop timer if at all possible. There are several to choose from including free (if you can solder) - see my signature.

Failing that, an electronic timer, not an inconsistent mechanical knob.

Failing that, a loudly ticking clock. I printed for my first year with a loud clock and a table on the wall listing 1/4 stop timing increments. I would take the ticking clock over a gralab any day.