Yeah, I don't see this working.
It works like this:
1) New members get one domain for a year per 400 posts.
2) Users that have more domains than they would if they registered after the rule was activated (ie. one domain & 200 posts, or two domains & 500 posts) get to keep their current domains, but their post cost resets as of now. So for the first example, the person will need to have 600 before they can register a new domain / renew their old one, and the second example needs 900 posts.
3) Users that have less domains than they would if they registered after the rule was activated (ie. zero domains & 300 posts, or one domains & 600 posts) will be treated like new users. So for the first example, the person will need to have 400 post for their first domain, and the second person will need to have 800 posts before they can register a second domain or renew their first one.