A metronome (or other audible timer is the only way to go to get exact repeatability. And use a card to uncover and cover the print. Never have the timer do it for you. I wrote about this in an article "On Printing", which was originally published in View Camera magazine. It can be found at www.michaelandpaula.com under "Writings." Although I use a metronome for contact printing, the same principles hold for enlarging. Use of an audible timer will cut your darkroom time by 30%, more or less, with more consistent results, particularly when dodging or burning.
Michael A. Smith