I’m on my first cycle of 5/3/1. After my deload week how should I reevaluate 90% of my 1RM for the four major lifts? Should I use the last set of that cycle or take a day and find my max?
No you should just add max 5pounds/2,5kg on the trainingmaxes( 90% ) for press and bench and max 10pounds/5kg on the traininmax for squats and deads.
Do yourself a favor and read the book, it will answer 99% of all questions possible and its some funny bits
in there as a bonus.
Good luck dude 
Its not your training max, its your actual max you add the weight to.
I’ve been adding 5 lbs to my actual lifts.
Maybe that’s why I stall out after 3 cycles.
[quote]Ares013 wrote:
Its not your training max, its your actual max you add the weight to.[/quote]
It actually doesn’t matter because it will come out the same. If you add the 5 lbs to your actual max the training max goes up by 4.5 lbs. It only makes a 1/2 lb difference in the calculation.
Add 5 pounds to your actual maxes for overhead press and bench press. Add 10 pounds to your actual maxes for deadlift and squat. Calculate your new training maxes from your new actual maxes and your working weights for the cycle from your new training maxes. Come on, even the free 5/3/1 article on this website says that.
Get an excel file and make your lift much easier. The one I have just adds 10 lbs to my training max. Don’t max out after every cycle, that would be stupid and pointless.