Ha yeah.  I got the sense from reading his 5/3/1 book that the idea behind using that metric you're talking about was so that you can have a way of setting a “PR” every time you train and thus plan your sets and reps out within his program scheme accordingly.  When I read it, it seemed like more of a way to map out the program so that you can choose to either “improve” every time or vary your intensities within each three-week cycle while working within the required percentages and so on.  If that sounds like gibberish, look at the 5/3/1 manual and you'll see what I mean.  -Adam