2mm is where it starts to get too loud for my taste, though my general answer to this question is "get it exactly right." Measuring with calipers works pretty well, but you are assuming that your frame is perfectly aligned. Wooden yardsticks are guaranteed to be off at least a little. I have a good steel carpenter's square that I use by laying it flat against the chainring, then I check against the calipers. There used to be a park chainline gauge, but they don't make it any more and it was kind of jingus anyway.