From HB2ASB, JN36pt

I think it's impossible to calculate meaningful ERP values from field
strength measurements from LF amateur radio stations. The measurement
uncertainty is just too large.
Already on VHF, ERP measurements are difficult, even in a controlled
environment.
As an example, look at the ETSI standards and see what they say in the
chapter "measurements uncertainty". You will often find +/- 6 dB for the
radiated power ( for comparison: conducted RF Power +/- 0.75 dB)!

73 de Toni