Something on the nose is right where it should be or exactly correct. If you guess someone's age on the nose, you guessed just right.
When it's 5:00 on the nose, it's 5:00, not 4:59 or 5:01. A bull’s eye means you hit a target on the nose. If you guess the number of jelly beans in a jar on the nose, that means you’re exactly correct. Well done! Anything on the nose can also be called “on the button” or “on the dot.”