Here's a utility that lets you precisely measure a tempo in beats per minute.
It can even be used as a dynamic metronome.
Tap out a beat on either the beat button, or the up button, and at the top of the screen you'll see the beats-per-minute, the number of beats since your first tap, and the elapsed time between your first tap and your last.
When the Metronome Mode box is checked, you only need to tap out a few careful beats at the beginning, and you can hear if the palm's beat drifts away from the beat you're measuring. When this happens, just tap along with the beat you're measuring a few more times to brinq it back on track.