Just FYI, this is making them stricter. Right now the requirement is 1 Gr 1 win and 10 total SWs.
So what I have thought of doing is a flat % on what horses you’ve bred have won. Say, 1% of what horses you’ve bred have won for up to 20 races (or perhaps 2 racing years, whichever comes first). The trick, of course, is that if you sell your future SW to someone clueless who spends 2 years racing them on all the wrong surfaces/distances/etc and they earn nothing, you get nothing, even if they end up going on to win millions when their owner becomes non-clueless.
(The other trick is that this money would basically come from nowhere, which kind of bugs me from a reality perspective. Perhaps FF could have a pool of money from entry fees, though, and it could come out of that…possibly even to the level of detail like Social Security…you get out what you pay in. So if you have a string of 30 racehorses and spend, say, 100k/yr on entry fees running them, you can get up to 100k back on the other 50 horses you bred and sold who are running races. If those 50 horses are extra amazingly awesome and you “should” get 200k as your breeder fee, though, you don’t get that extra 100k unless you spend it in entry fees.)