AltairLabs:
Here is freq counter we use for DCDW, its speed is determined by the incoming freq and the number of digits of precision you specify. It also has code to reject noisy signals. Probably slower than you want and too many features, but you might find the autoscaling approach useful. There is a fundamental limit on precision vs measurement rate and this is one way to approach it.
That code looks interesting but it's not Arduino ready is it, no setup, loop, Serial.begin(), etc.
I tried adding stuff to try and run it, but gave up after two or three fixes. Compilers are so finicky. 
Lefty