Charles & Sandra Cohen
You don't have to calibrate to "zero beat":
. . . You can calibrate to _any_ audio frequency, by offsetting the receiver's dial setting.
If you have an Android or Apple smartphone, you can download a free "guitar tuner" app from their respective stores -- I use "DA Tuner", for Android.
The guitar-tuner software, running on the phone, will give tell you the frequency of a pitch it hears, to within 0.1 Hz. It's sensitive enough to exhibit "Doppler effect" if you move the phone toward, or away from, the rig's loudspeaker.
I just gave a talk about this to my club.
. Charles / VA7CPC