I have my own rating system which I run on WA races and I place my bets according to which track the races are at. I find for instance that at Belmont over the years I can come out around 12% ahead year after year and at Ascot I come out around 8% behind year after year. Hence I very rarely bet on races at Ascot but always have a go at Belmont. I also find that some months perform better overall than others at most tracks, not really sure why, maybe temperature, weather, new horses returning from spells, I haven't been able to work out why.
My ratings are based on "form" rather than barrier such and such or third run back from a spell or whatever but I suppose it might apply to all sorts of systems.

KV