How long is a piece of string? Assuming all other things are equal finishing 5th. beaten say 1.5 lengths is better than finishing 3rd. beaten 4 lengths. Placings are used, particularly for systems as they are simple and easy to see in a form guide.

I devised a system some time ago which is still doing OK. A key rule was that the horse could not be beaten by more than 2 lengths. Sometimes we did allow greater than that if the horse had bad luck in running. However, that depended on the horse. Some horses regularly have bad luck in running because they simply cannot put their act together under racing conditions. They'll run into horses in front of them, the slightest check and that's it for the day etc.

There's also good rule of thumb. Winners keep winning and losers keep losing. By the time a horse has won 3 races in a row it can often become a good bet because of the "due for a loss" syndrome, so it goes around at overs. Again judgement has to be used.