You make figures for races with a lot of unraced horses when you have enough info to know you’re getting it at least close. That Info comes from all the runners, not just the winners, and it might or might not be clear when a few have run back once each.
This becomes less difficult as the year goes on not just because you have more data (figures) to work with, but because they start running distances older horses also run, making correlation with surrounding races easier.
