Assume your shooting stance with your gun low, close your eyes, mount your gun, open your eyes, and see if you're sight picture is down the rib with the barrel/gun pointing where you look.
As for length of pull, shoot whatever feels comfortable, but it's hard to "feel" it when it's only slightly too short. The best way for me is to keep increasing the length of pull until it gets too long, then just back off a little.
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