Crash, I've done years of research into Pre-post favs, amazing thing is the overall picture is surprisingly the same as for actual favs, i.e. S/R, LOT etc, even the picture when sorted by price, type of race, LSW etc etc. I'm talking about over 1000's of bets here, although of course as you say the actual bets can alter as sometimes the fav is not the pre-post fav.

I've found a slight improvement when the pre-post fav IS the actual fav, and also when 2 or more sources agree on the pre-post fav, various other filters can get it down pretty close to break even/slight POT.