That wasn’t Thibaudeau’s advice It was to hit a 2RM or 3RM, period. No calculations needed. And it was specifically when training for strength gains.
But for the better part of a century, lifters built plenty of size and strength training by feel, not working off a percentage of 1RM. That approach is relatively recent, and again it’s predominantly for lifters who prioritize building strength, not nearly as relevant when focused on building muscle.
Even then, for example, 5/3/1, which is probably the most popular and arguably most successful percentage-based program around, has evolved to not need a 1RM test at all and bases numbers of a percentage of your 5-6RM.
It all circles back to, why are you investing so much into knowing your one-rep max. Just for the sake of knowing it? That’s fine, just know that finding it high risk:low reward.