Nice one jakeluck, just tried the 20 / -15 mix on a couple of epubs and it does actually makes a subtle but noticable difference to the darkness with no real loss of quality (that I could see) and no background darkening.
mmmm, makes the custom font I slaved to get into the epubs even nicerer!
(have to use the tweak method for the custom fonts as there is no russification for us mac owners...
)