Remember an S unit is calibrated to be 6db and double or half power is 3db or – 3db
You have to QUADRUPLE (X4) your signal to DOUBLE your signal strength at the receiver end.
Likewise, if you drop your power by one-fourth, your received signal strength will be one-half less, or 1 S-unit. You are working a station running 100W and he is S8. If he drops his power to 1/4th, or 25W, his signal strength should drop about 1 S-unit, or to S7.
If he drops another 1/4th, to about 6W, he should drop another S unit, or to about S6.
Therefore, the difference between 100W and 5W QRP is about 2 S-units. Big deal.
Dropping to 1W is about another S-unit, then to 250mW another S-unit, etc. OK, now you’re getting down into the S4 noise level on 40M. Now you are really hoping the receiving station has a noise level lower than s4 in order to hear you.
I have been running propagation test on 20m using 250mw and reaching Europe and Australia into a vertical shows that QRPp works on CW or digital modes
Tmas Ve3FKN has also been running tests on 40m using low power with amazing results