Stu, I've been reading about this lately from Matt Perryman, what he calls Autoregulated Progressive Resistance Exercise, and which he attributes to Mel Siff, in Supertraining
. Here is one of his posts on the subject:
There are templates for 3-rep, 6-rep, and 10-rep routines. You have a constantly-changing "training max", for instance a 6-rep max. You do warmup sets then use the 6-rep max and go AMAP (he advocates leaving 1-2 in the tank). Then the max is adjusted up or down, or remains the same according to how many reps you get, and the new max is used for the last set. Then the performance on that set determines the "max" to use for the next workout. So when you can lift more, the max goes up. If you are having a bad day and don't get the reps, the max goes down.
There's something about this that's akin to 5/3/1, in that you do AMAP, so the routine is not a rigidly fixed thing, but flexes according to what you can do on that particular day.
This is what I was thinking about using when I was talking about making some changes a week or so ago. I'd like to be able to read Siff's book, as I hear it is very good in several respects. I would have used the 10-rep routine. By the way, in Matt's blog he only gives the table for the 6-rep routine as an example, but someone gives a link in the comments to an article that has the others.