Don't knit but what some call "casting" is when you go partway through with your second needle and then pull the thread over the needle before pulling it all the way through. Nigel's videos show the technique.
Depends I would think on the specifics. Thickness of leather, layers, etc. all come into play. For example, I usually use around 4 oz. leather for wallet backs and I add 1/2" to the outer piece as compared to the inner.