Buy a metronome?

Nah!

Go here:

http://www.metronomeonline.com/

the number is BPM - Beats Per Minute. Set a comfortable beat (120 BPM is what most popular music is set to) and adjust your macros to it, or vice versa.

As a very amatuer musician of many years, I can maintain 120BPM on my own, just tapping my foot.