If you're looking at horses on the limit, I tend to shy away from those ridden by claiming apprentices. Why? It seems nowadays most apps (I know I'm generalising) can't ride below 53kg or 52kg anyway, so they can't claim their full allowance, if at all. Bit of a waste.

Again, as a mechenical rule, I also take notice of first-uppers & 2nd uppers (or first starters & 2nd starters) over any distance. Its a general rule of a lot of systems to eliminate these horses automatically.