If you ask an inexperienced trainee how hard his work set/workout was, against what does he have to measure this subjective assessment? If you propose that he base his next workout on this subjective assessment, what have you done to his next workout? "RPE" is a useful tool for advanced lifters, used in appropriate ways. For novice and intermediate lifters, it is an excellent way to completely derail the process of linear progression, because it doesn't matter how hard the set "felt." Perceptions vary. Stress drives adaptation, if you recover. That is all. Let's leave RPE out of the app, okay? That's Mike Tuschcherer's method, not ours.