Dear Bodean,
I hate to say this, because it is contrary to what many people believe and expect, but you really don't need a metronome. You should just relax and let your putter hang from one arm and lift it out to the side and then allow it to free-fall past your hip whil ekeeping the arm "dead", and watch and learn. Each person and each putter constitue a slightly different pendulum, with a different BASE tempo. What you need to know is your BASE tempo, and then you can take it from there.
The BASE tempo is what tempo gravity ALONE gives your pendulum system (body, setup, and putter). Typically, for an adult male of usual stature and a conventional putter, this BASE tempo is close to 60 beats per minute. That's just plain physics on earth. But to actually see YOUR BASE tempo, just watch the putter swing. You could time it if you want to, but watching is probably more exact and educational.
If you set a metronome to 60 and then let your putter swing, see if it makes it all the way from start to finish at the other side in one beat. If it falls short of reaching the end when the beat comes, decrease the metronome setting a few beats and watch again. If the putterhead gets there early, speed the metronome up a few clicks.
Once you have this BASE tempo, try a few ten footers. You'll likely want to go a little faster, so experiment by increasing the metronome a little but not by 5s or 10s. Many pros use a nice 65 bpm setting, and not many get far above 75 bpm. I think Loren Roberts looks like about 65 bpm.
Incidentally, the business about "one potato, two" is to coordinate tempo a little with breathing and to suggest a flexible yardstick suitable for different people.
Cheers!
Geoff Mangum
The PuttingZone
http://puttingzone.com
Golf's most advanced and comprehensive putting instruction.