This helicopter is a military Black Hawk. In this case it's flying with its Air Traffic Control transponder on and flightradar24's track shows it heading to Concord and passed almost directly over my house.
I derived the equation, it appears to not be well known, and it gave me a surprisingly good match to the helicopter's speed.
Note that at low speed (fa – fd) will be small and that's why the equation gives a low speed.
If a helicopter could get close to Mach 1, fa becomes very large and fd approaches ½ of the base frequency so the fraction becomes close to 1.