I postd a bunch of calculations but the post got deleted. If you're basing your weights off your 1RM, you'll end up overshooting the weights by using the 100% BW calculation. If you're basing backoff sets off your top set of the day (for example, 95% of your top set), you'll end up undershooting the weights by using the 100% BW calculation.
The difference between using 100% BW and 85% BW is small though, about 24 lbs. With 5/3/1, it probably won't matter, since you're undershooting your 1RM anyways. If you're working with higher intensity and lower reps, the difference could mean a missed rep or an extra rep, grinding when you didn't mean to grind or a set being easier than it's meant to be. I don't think you need to be this accurate with the lower body, but I find that being more accurate with the upper body tends to help.
Some food for thought...
