Ive seen varying freq with Schumanns name next to it but no calculations to back it up.
The earth has 21600 min of arc and Nautical miles or Geographic miles at the equator.
Theres officially varying data on distance too which keeps the peasants in the dark (TAX PAYERS ,in modern language)
Geographic mile 6087.2 feet =1855.3786 metres , x 21600 = 40076177 metres
International N mile 6076 feet =1852 metres , x 21600 = 40003200 metres
Brit Admiralty mile 6080 feet =1853 metres , x 21600 = 40024800 metres
These are only 3 examples ,theres more and then the astro guys have their measurement too.
At this stage the Geo mile is the largest so it has to be accurate enough.
Light speed /Wave length = freq
At the moment official C speed =299792458metres sec
299792458 / 40076177 = 7.48 hz
If light speed is constant and the equator is constant then the freq is fixed ,a standing wave