ZCS get a lot of bashing for estimating that XX is better than Z3F. Recently, I remember a passage in LOCH and OTG that might give ZCS some cred back.
In LOCH, when MCF witnessed the fight between HYS and ZBT on Peach blossom island, she was amazed of the level of MA they displayed. She never thought anyone could wield such power or possessed such amazing MA.
Now, why on earth was she so amazed? Surely HYS must have demonstrated awesome feats during a training session? This shouldnt be a surprise. But she was, and that led me to draw the conclusion that people seldom go all out during training sessions. So the students never get to she their teachers full capacity unless they see their teacher in a fight.
In OTG, XYK (the guy with the iron badges) was going in the woods to practice. He thought he was alone and decided to go all out (I got the impression that this was something he seldom did). Unfortunately, he wasnt alone. A few people showed up and XYK regretting that he went all out, because this almost depleted his powers. Luckily for him, the people saw that he was practicing and though that his display of power was him casually practicing (because nobody use 100% when practice?). So they were afraid of what he could managed if he DID go all out.
ZCS wasnt that powerfull, so perhaps Z3F only used his left arm and 30% to teach him what he needed and could managed?