I use the following system , which can end up more of an art than science.
First I look at the the deduction factor ( which I use as the PP)then the the actul fav with the lowest odds and the horse with the highest amount of money on it. When a horse has all these stars aligned, it is obviously the favourite.Same for second fav and third fav. Things get interesting when one horse has, say, the lowest odds, another has the highest reduction factor and a third has the most money on it. then I have to make a personal call. BTH all this only takes a minute as I only look at a race for the first time 3-4 minutes before the start.
Regards