So, your 1-rep maximum for bench press based on the number above is around 260 lbs. Then, you take 90% of your 1RM (so, if your 1RM is 260, 90% of that is 234) and then use that number as your "training max." Your training max, being 234, is then multiplied by 65%, 75%, and 85% for your first week on 5/3/1. As you probably noticed, the percentages and number of reps change for weeks 2, 3, and 4.
So, your first week on your bench press day if you are doing The Triumvirate (using the stock assistance lifts) would look like this:
Bench Press
Warmup sets
5x152 [(260 x 0.9) x (0.65)]
5x176 [(260 x 0.9) x (0.75)]
1x199 [(260 x 0.9) x (0.85)] (on this last set, you get as many reps as you can with 1 being the minimum)
DB Bench Press
5 sets of 15 (at the heaviest weight you could handle at this volume)
DB Row
5 sets of 10 (at the heaviest weight you could handle at this volume)
I think you've got the right idea, but I figured I'd repeat stuff just to double-check. You're basically correct in that you can adjust the assistance lifts to your pleasure as there are a ton of good suggested assistance lifts in the book. I added lunges and leg press to my squat day (which, by the way, is entirely painful, but so worth it) and front squats to my deadlift day.
