Beaten margins

One notices that at certain tracks, finishes are often bunched, and it would appear to be just luck, that one of half a dozen less than a length apart, manages to poke its nose in front, where the line happens to be. I am thinking these kinds of situations, don't really favour the shorter end of the market, and one can perhaps extend the odds range a little bit. On the other hand, there are other places where they regularly come to the line more spaced, and perhaps favourites should be the focus. Naturally, allowance has to be made for going, distance, and possibly the class of races, etc. Facts and figures can be assembled to analyse this theory, but firstly I want to establish a good metric, that tells me where a particular track/dist/going sits on this scale. My initial thought was the beaten margin of the third horse, I also considered perhaps the median beaten margin of the field, what do you think would be a good index ?