1) Select a predertimined Win% (can be the Win% the horse would be, after / if it won) .....

2) Choose a minimum number of race wins - this excludes horses who have had say 2 starts for one win etc, etc.

that's it .....

Over a period of time, you will notice that the Win% declines progressively to about 20% ..... before the Win% is too low to qualify

A couple of commonsense filters and away you go.