Converting pulses to RPM
You should first calculate the error that each method would produce. There is so called frequency measurement and period measurement. Some MCUs have built-in a special unit QEP, QEI,.(manufacturer specific names) that switch form these two methods to give best results.

frequency measurement:
Within a fixed period of time you measure the incoming pulses. The relative measurement error is \$\varepsilon = \pm \dfrac{1}{N}\$, where N is the number of counted pulses

period measurement:
A measured signal is used to start/stop counting an internal clock generated pulses. The relative measurement error is  \$\varepsilon = \pm \dfrac{1}{N}\$ where N is the number of counted pulses.


The clever way of solving the problem would compromise of using both methods and then compare the relative error, then switch the method of measuring with regard of the lower error. Alternatively you could compute these errors based with known input measurement signal frequency and 1st fixed measuring time, and 2nd fixed internal clock generator frequency. You do choose which method gives the lower error.
For high frequency measurement the method 1. is more suitable, for low frequency measurement the 2. method is more suitable.