Nice comment JFC, it's nice to discuss real form for a change!

I reckon a good starting point is to look at races where two horses have run against each other a few times in the recent past. Check the weights they carried and the lengths they finished from each other (2 lengths to 3Kg is some sort of conversion factor that gets bandied about from time to time [which I reckon is not very accurate]). See if there's any logic to the change in weight and the change in margins. There will be other factors contributing to the difference of course - like if one of the horses was first up it might improve against the other as a result of being fitter for the run for example.
Checking out these comparisons will give you a feel for what might affect a horse's performance against another horse from race to race and more importantly it will make you throw your hands up in the air a lot and say "How the frig did that horse improve/deteriorate so much". It will give you a feel for how logical the form sometimes isn't.

KV