Holding the pick is an idividual thing..... probably everyone will give you a slightly different answer.

For fast alternate picking, I leave a bit more than 1mm of the pick sticking out past the end of my index/thumb. I angle it slightly, so the edge of the pick nearest the neck is slightly lower than the bridge side.... not too much though.

Works for me - but some players use a different finger/thumb combo, and many vary the amount on angle on the pick...