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It's pretty simple. Just make another field to store the new referal number in. Then add a cron job to update the number when users that are refered reach a certain post count.

Otherwise another way would be to make referals only count people in a certain usergroup. Make a base usergroup, then another group for those who have the minimum number of posts. Use the promotions system to automatically move people then.

Both of these ideas would need to be coded though. The second one makes more sense I think, and would be much easier to implement since it wouldn't require adding any new tables. Might have to be a code hack though, since you'd have to turn off the referal code within the register.php file and add it somewhere else.