In British English, "he's quite a nice guy" (70% intensity) and "he's a very nice guy" (100%). Hope that helps!

However we can also say "he's quite the man". This means he's the "top guy", which is a 100% meaning! But the first usage I indicated of 75% meaning is much more common. Hope you're not too confused now!

The difference depends on where the English is being spoken. I can't answer for British English, but in American English, it's rare to hear someone use "quite" ever. We use "very" "really" and "pretty" in these situations. For example, scoring 17/20 would be "pretty good" at math; 19/20 would be "very good" or "really good" at math. "Very" is more formal, whereas "really" is more colloquial. PS. The only times I would use "quite" is actually to pretend to speak like a British person... that's how remote the world is in American English :P

Well, if "quite" means completely and it is less intense than very. Then it somehow means "completely but up to certain possibility". Very is something like, when we differentiate between high and low, when we mean something really is going at high level. e.g He plays Football very good, ______ is a very corrupt place e.t.c

Maybe instead of completely we could use "truly." Quite doesn't mean extremely, but it is definite. Sometimes we use pretty in a similar way. He's pretty good, or it's a solid score. He is actually good, and not just a tiny amount. Very, though, is extreme. Yes, if using them as qualifiers, quite would be less than very, but more than a little.