"fred harris rule of thumb":
You can find the formula for (at least) one "Harris method" or "Harris approximation" in Multirate Signal Processing for Communication Systems, Fredric J. Harris, 2004, page 216, equation  (8.16), with details in Chapter 3. F. Harris calls it "harris approximation" (without capital for the name), namely:

$$ N = \frac{f_{SMPL}}{\Delta f}\frac{Atten(dB)}{22}$$

Also in Digital Communications with Emphasis on Data Modems: Theory, Analysis, Richard W. Middlestead, page 752, equation B.39.

Former questions might provide you with additional insights and references:


How many taps does an FIR filter need?
Filter Order Rule of Thumb