How could Deku hit 1000,000% One for All punch when he had no memory of it? In his fight against Bloodthirsty Muscular, Deku hits a 1000,000% One for All punch when we haven’t seen even All Might use that much power. How can this be possible?
It was confirmed by the Author that it wasn't actually 1000,000%. Deku said that to pump himself up.

This is explained in this article, as well as the image below: