I think back computing the power level required to generate 3mV at a given spot needs a lot of experience of the Millington Method and conductivity atlas.

In my own experience, the 648 signal in Fife, even on the SE and E beaches around Anstruther and St Andrews, never get above half a millivolt/meter. That however can be more than sufficient for good reception, given the very low background noises in most areas of the Kingdom. The only fades I've noticed have been after sunset.