It’s never been easy to accurately determine the maximum amount of weight you can lift for one rep.
You either expend a ton of time and energy as you lift, rest, add weight, lift, rest, add/reduce the weight, etc. – judging, missing, and guessing your way throughout the haphazard journey. This cycle continues until you find a true one-repetition maximum (1RM). The process, at best, will take 15-20 minutes per exercise and it can be exhausting – literally and figuratively.
Or you perform a submaximal set to failure and plug your numbers into an equation that may, or may not, have been clinically tested. Furthermore, many indirect methods (i.e., submaximal lifts) that calculate a 1RM require you to fail at a specific rep, say, rep 10 or the calculation won’t work.