Commonly and also acceptably defined in terms of the overlap (or 'valley') between two peaks. Thus for two peaks of equal height, masses m 1 and m 2, when there is overlap between the two peaks to a stated percentage of either peak height (10% is recommended), then the resolving power is defined as m 1 m 1- m 2. The percentage overlap (or 'valley') concerned must always be stated.
PAC, 1978, 50, 65. 'Recommendations for Symbolism and Nomenclature for Mass Spectroscopy' on page 72 (https://doi.org/10.1351/pac197850010065)