We have all at some point heard about the 80/20 rule, more formally called the Pareto Principle, is usually stated something like "80% of the work takes 20% of the effort". The corollary would be that 80% (16% of the whole) of the remainder takes another 20% of the work, 80% of what's left will take another (3.2% of the whole) takes another 20 % and so forth - in other words you'll be running afoul of the law of diminishing returns fairly fast.
If we consider that shaving consists of
- Prep or pre-shave
- First pass
- Second pass
- Cleanup or buffing
- Post shave
and further assumes that each part takes roughly the same time (which holds true for me at least)... we can quickly estimate that a two pass plus cleanup gets rid of 99.2% of the stubble. A three pass plus cleanup will get a little bit more, but unless you did a sloppy job on the two first passes the difference should be quite minor.
So there you go; a tongue-firmly-in-cheek "proof" that a two pass plus cleanup gives a Damn Fine Shave!
As a sidebar: If you find that 80% of the effort do 20% of the work when you shave, it's time to change your blade or strop your razor.