I'd just like to ask all the people that do ratings, how do you decide on what is the qualifying run?

I'm just starting to get interested in doing ratings again but I'm looking at keeping everything VERY SIMPLE. Looking at some of the results over the last few days, I think it's possibly a good idea to simply use the horse's latest run, or perhaps it's best rating run out of it's last two starts or something. But what about horses resuming after a spell. What do you do with them?