Whenever I have to reset or start from scratch (injury - etc) I work up to a five rep max. Then I take that number minus the number of reps - as my max. Ex: Way back in 2008 when I first started 5-3-1, I benched 305 x 5 reps - and thus used 305 for my max. I then took 85% of that as my training max and began.
After trying many versions/ techniques, etc - I find that NOTHING works better with this program FOR ME than simply setting a record when I go in without compromise - even if it is only by percentage points.
The trick to doing this is calculating and writing down what you need before you go in and simply doing what you gotta to get it. Thus I take getting to the gym ready to compete (with myself) seriously.
So if I need 335 x 11 to beat my last workout record by .0001% - I am not stopping until I get it - but it involves planning and peaking weekly.
Of course this runs its course - but then you realize via experience that you got stronger using sub-max weight and thus the training max becomes just a tool to regulate. So then the 5-3 element makes perfect sense.
I know that Jim and many others who know far more than me say you don't have to set a record every workout - and especially when doing BBB I agree - but compromising the PR aspect of this program has never served me well.
So In short, start light enough that you can get started setting some records, get some momentum....