This calculator can be used to compute cutoff frequency of Betaflight 3.3 RC1 Kalman filter (which is BiQuad+FIR2 not Fast Kalman but this is a minor detail).
Since the filter is looptime dependant, you have to provide Q and R coefficients as well as gyroscope sampling frequency. As a result you will get computed cutoff frequency. Play with Q and R values to get cut off slightly below motor noise frequency of your drone.
Brought to you by QuadMeUp.com and Paweł Spychalski YouTube channel