The method of gingertipster is the way to go. If I can find them I will report back but a few years ago I rated over 3000 horses and determined an edge %. For instance if I rated a horse 3/1 (25%) I needed 4/1 (20%) before I logged the selection as a bet and in reverse if I rated a horse $20 (5%) I logged them as a lay at $10 (10%). Both methods did ok without being startling however I did not want to do the form on every single runner and then price them all and then be hunched over a screen all day nor did I want to spend time learning Betangel and any other similar setup. It just did not fit into my personal family/wife life. There was also the really annoying times when a track was rated good on a Thursday yet on raceday conditions changed.
These days I find it easier, though “easier” is the wrong word, to just pick best bets, follow a trainer or two at the right odds and back the odd roughie that I feel the market has wrong.