Much is made of finding exoplanets in habitable zones, locations with orbital semi-major axes permitting water in the liquid state. Habitability may be compromised if such bodies become tidally locked, orbiting within the star's tidal lock radius. The illustration below shows the tidal lock radius for various stellar spectral classes:
The diagram shows Mercury within the tidally locked zone although its really in a 3:2 spin-orbit resonance. How is the tidal lock radius calculated?