Page 280 "The more charges are tapped at once, the more noticeable these changes become." should be "The more charges that are tapped at once, the more noticeable these changes become."I think the "that" you included is not necessary (maybe even wrong?); "at once" is definitely weird there, though. Maybe "simultaneously" or "at the same time" instead?
Well, 'once' just means "one time" or "a single instance", so it could be written "at one time" or "in a single instance", which sounds correct to my (admittedly, Northwest-to-Midwest U.S.A. English) ear. Your experience may differ.
Perhaps the 'that' is unnecessary, but then I would feel obligated to remove 'are' as well, making it "The more charges tapped at once (or at a time, etc.), the more noticeable these changes become."