Wesmip,

If your figures are valid then I advise you to take your loss-chasing (aka "progressive") plan and hurl it as far as possible off Bulli Pass - or equivalent.

Then analyse your figures to try and find a filter to get you profitable on favourites at level stakes.

Your experiment suggests that Betfair Markets have a memory about favourites. Leading to overbacking when favourites are running hot, and/or underbacking when they are in a slump.

So something like only backing favourites following n outs will probably be the optimum strategy.