I don't think you can tie it to anything. It complicates things considerably. I think the easiest approach is to just let buyouts of contracts immediately come off the salary structure of your team. As in if player x is bought out with 2 years left on his contract it no longer counts on your cap figure. If you cut a player his salary counts 50% towards your cap that season and is removed whenever contracts role over for the next year.
I think arbitrary stats like +/- or per or whatever you want to use is never gonna fly in a contract negotiation. Even current standards and ethics clauses in contracts are a major hassle to enforce. How do you prove Eddy Curry didn't perform up to Eddy Curry's standard? You can't. Plus if you ever use something like that it sends a bad message to other players. It should be about lessening the hit not removing it.
Maybe a clause that permits the release of one contract per team every year or two-kinda like a reverse MLE- would help also. The key to that being every team gets to use it not just over the cap teams.
