16-bit timer of ATMega: How to calculate timer count?
The frequency of your timer interrupt will be equal to the following:

$$
\frac{\mbox{microcontroller clock frequency} }{(\mbox{prescaler}) \times (\mbox{compare match register value} + 1)}
$$

So, since you mentioned that you're using an Arduino which comes with a 16 MHz crystal, to achieve an interrupt frequency of 24 Hz the value of the compare match register (OCR1A) should be 650, given a prescaler of 1024.

If you want to change the BPM, you'll have to set OCR1A on the fly.