The eternal question! I think it's average prizemoney won in the horses last three months RACING (exclude spells), with it's biggest payday deducted from the calculation.This tells you in one hit that the horse has been doing better than the others RECENTLY(or up until it spelled) and the numbers are not distorted by some fluke "big one". If that day was not a fluke then there were others to bolster the average.
The only proviso is that today's race must be a "suitable" distance and ......"suitable" prizemoney (ie at least it's average win amount). The reason for this is that we don't expect Makybe Diva to win at 1400 despite having an average far superior to the others. Likewise I don't expect a horse to be really "sent" to win a $25000 first prize when it's been cleaning up in million dollar races. That looks too much like "practice" to me and I'll wait 'til the trainer is serious.Cheers