Paul, check the track stats on racing and sports website. They have winning stats for the position of the horse 400m out. Races 1200m and less seem to heavily favour horses that are positioned in the first 4 at the 400m mark for most tracks. Tracks with longer straights such as Rosehill, Randwick, Flemington and Sandown, for example will favour back markers a little more, especially as the distance gets longer.
It can also help if you can figure out if there will be lots of early pace in the race. If there is, your backmarker can swoop late as the leaders tire at the end.