G4ILO Whispering with 20mW

Yesterday Julian G4ILO wrote on his interesting Blog about his advantures with WSPR with low power:

Many people who operate the weak signal mode WSPR use too much power. 
If you don't use very low power (QRPp) you'll never find out what this mode is capable of. 
So to remind myself I thought I'd do some whispering on 10m using 20mW of power. 
It's easy to use low power with the Elecraft K3 as the power level is adjustable in 0.01 watt increments. 

The power of 20 mW that julian used to WSPR can be compared with 400 mW in CW. (13 dB --> 20x)

Today I visited the WSPR database to make the following analysis.
In each spot I calculated the lowest power what could be used, to be received with a SNR of -29 dB.
The stronger the signal, the lower the lowest possible power.
The stations that received the strongest signals are ON7KO (600 km), LA9JO 1900 km) and ND6M (6600 km)