red lion rotor tach

[Penguin]

cesphil - your calculations are correct.
But using the doubler function (post #15) changes the sampling rate which gives less accuracy but less latency (more responsive). Not using the doubler gives greater accuracy but more latency.
Given that the update rate is less than a second, I don't think latency is important.
Using the doubler, you may notice some jitter in your least significant digit.