This has been adequately covered, but let's count several ways.
Let's say that one month my TM for OHP is 60 kg. On the 5/3/1 week I press 57.5 kg for 6 reps.
Two months later when my TM is 65 kg I press 57.5 kg on the 5s week (truth be told I'm not sure how you can say you're never lifting the same weight twice). I get 7 reps. New PR!
Or let's take the month before that. With a TM of 62.5 kg I'm pressing 60 kg on the 5/3/1 week. Let's say I press it for 6 reps. 57.5 kg for 6 reps to 60 kg for 6 reps. New PR!
Or use the calculator in Jim's book, as he stated in the post above mine.