When dealing with exposed horses running in handicaps, the odds the horses started at last time adjusted for how they fared gives you a good starting point. It's an extension of the Iterative Principle - the trade off between expected outcome and actual outcome - which is also seen in handicapping.
As an example, a horse starting at 10/1 and finishing fifth in a 10-runner handicap might start around the same price in a similar race next time. A horse starting at 33/1 and finishing fifth in a 10-runner handicap might start around 16/1 in a similar race next time. A horse starting at 4/1 and finishing fifth in a 10-runner handicap might start at 13/2 in a similar race next time. Etc.
That is, of course, before considering such things as fitness, changes in trip, drops/rises in weights, luck in running, and, perhaps most importantly of all, whether THEY wanted it then or want it now.
But it's a starting point. The market does not always get things right, but a horse's odds last time is a distillation of an awful lot of information that just needs to be put into a different context today. In some instances that is not all that difficult.