You are overlooking so much of the program it is hard to know where to begin.
The "amrap," the principle of the "training max," the 90% factor, the 5-3 deal, the .03333 formula, etc... they all work together. I have done BBB challenges and all of the permutations of it - I like it a lot. But when I first started I used the Triumvirate (like has already been mentioned) and milked that for a long time. I also used amraps on the last set as much as possible because, quite frankly, it's fun as shit.
I have had to get smarter about it all over the last 5 years or so - but it's still my favorite part of the program. I even did them when doing BBB, mainly because I am addicted to beating my own records and I am also not the sharpest tool in the shed.
I get why it is not advised though
Also, strength progress is very rarely linear and logical. Have the plan and use it as a guide but be prepared to adjust as needed. I doubt if your calculations will happen in the way you expect. The human body has a mind of its own.