A basic question on pushups...

A common question is: how far to go down. I know the answer to that one: until your chin just touches the floor. But I would like to know how far are you meant to go up? Until your arms are locked straight, or just before? I recently noticed I push up even further than this with my shoulder blades, "shrugging" my body weight as high as possible.

I know different people will do different things, and slight variations in technique can be a good thing, but what would the text book answer be? How far are you meant to push yourself up with push ups?