Rule 1: Operate on handicap races.
Rule 2: Consider 3YO, 4YO and 5YO colts and geldings.
Rule 3: Top three in the pre-post betting.
Rule 4: Finished in the first four in its last start.
Rule 5: Must now be ridden by a claiming apprentice.
Rule 6: Last start <=15 days.

Rule five (the "claiming apprentice" rule) is the most important rule.

You can test and use as you want.