You can leave it in any mode you like, depending on your style of shooting.
Many people use Av as a default. Some would rather stick pins in their eyes than take it out of M. Some would never dare move away from the green box mode.
Av is a metering mode? :shrug:
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It should be 18% but I've vague memory that I read Canon had adjusted this figure but not revealed to what exactly.
The old back of the hand trick still works fine though
Mainly in Evaluative, but changed when circumstances dictate.