I ran my homebrew propagation model on the great circle from
DK7FC in direction of Todmorden.

Here's the received signal for the two frequencies,

http://abelian.org/vlf/tmp/171017c.gif

given 200mA current and antenna effective height of 20m.

Todmorden is at 881 km and the rxed flux density is

6430 Hz: 0.032fT
6510 Hz: 0.046fT

so 6510 is about 3dB stronger than 6430.

The model uses the international reference ionosphere

https://en.wikipedia.org/wiki/International_Reference_Ionosphere

and takes account of date, time and bearing. It is ray tracing
ground wave and in this case up to 18 hop sky waves. Actually
12 hops seems enough.

--
Paul Nicholson
--