During daylight hours my RST would be 559, or even lower.
Please notice that a power of 500 mW is only four S-points weaker than 100 Watts.
See the PA1B QRPp page.
Soon I discovered, that when darkness fell, the signals from the south of Europe would be very loud. So this made it easy, to make QSO's over a distance of 1000 km or more.
At that time, I did not realize, that my own QRPp signal, could also be an earsplitting 599, until I got genuine report from Italy, that my signal was S9, while using just 500 mW.
When the D-layer disappears during the night, the signal can reach the the E-layer without attenuation in the D-layer. The only attenuation of this sky wave, is (only) 6 dB for a doubling of the distance.
So the signal will be very strong.
I was wondering, what the lowest possible power would be, if I would reduce my power in that situation with this extreme propagation.
If I reduced my power, so my RST would be 559, the difference is 4 S-point. This is a reduction in power of 4 x 6 dB = 24 dB But also 4x4x4x4 = 256.
So I could reduce the the power from 500 mW to about 2 milliwatts.
From 2001 on, I made many CW contest QSO's over more than1000 Miles with 2.5 mW and 5 mW,
using homebrew attenuators.
I discovered that the frequency must be absolutely clear, when you use very low power.