These boundaries are tricky business. Alyx posted a lot of good stuff about enforceable boundaries in a reply to Glenda's Violation of Curfew thread on Nar-anon.
I have never said to my DH that he had to leave if he was actively using, b/c I knew it was something I could never enforce. In terms of his using, I couldn't determine or plan in advance what my breaking point would be. If he were still using, I think when enough was enough, I would just wake up one day and say, "That's it, I can't take anymore, I'm out." Then, once he made it into recovery, I felt that as long as I knew he was actively working his recovery, I could deal with slips now and then b/c I know they happen - it's reality. This boundary business isn't about dictating his using and his recovery. It's absolutely knowing what you will and won't put up, and sometimes you might not know when enough is enough until you actually hit that bottom or breaking point. JT has said it really well, "I may be living like this now, but I won't always live this way." (She said it much better!
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BUT! For those who absolutely know you cannot live with your SO's taking another drink, then the answer is clear. If he slips, drinks, relapses, whatever, then he has to go or you have to go, if that's the boundary you plan to stick by. There shouldn't be any exceptions, b/c then they won't respect you for changing your mind, and they will always look for that loophole or a way to get around your defenses. If that boundary seems to hard to stick with, then try to come up with something else you CAN live with.