I'm not really happy with this calibration,
I'll have to see what I can do to improve it.

Zero-beat to WWV is hard to do better than 50Hz by ear. Mechanical
aids will get you a little better but then at the few Hz level you hit
variations in carrier due to ionospheric variation.

For a few hundred $, HP Z3801A's are available on the surplus market.
They're a 10MHz OCXO locked to GPS. Short term Allan variation is
10^-12 or better over 1-100 seconds. Many lab counters and a lot of
ham radio frequency counters will happily accept the 10MHz reference
that the Z3801A makes.

Other telecom-related GPS-locked OCXO's/rubidium oscillators are
available on the surplus market too, some make telco-related reference
frequencies like 1.544MHz or 19.6608MHz which can be used to calibrate
on.

Tim.