The thing is about the zipper rule is that it can genuinely be difficult in practice sometimes. If it's a simple example of a busy slow moving two lane road becoming a one lane road, it's absolutely simple and unquestionably the best practice to merge late. But in other situations, especially ones involving exits and junctions, it can be down to judgement.
In very low density traffic on high speed roads, waiting until the lane is actually closed to move over can create slow downs, but moving left earlier doesn't. The point where the optimum behaviour changes isn't always clear and can change rapidly.
In the UK, the rule is that in slow moving congestion you should late merge but in other cases you should stick in the left lane unless overtaking. There isn't a specifically defined point where one behaviour should take precedence over the other.
