hmmm, i personally don't consider anything to be a "norm" -because i like to look at each shirt (or design for that matter) as an individual.
what works for one design, won't necessarily work for another, ya dig?
but what i DO use as a standization is the rule of 4 or 5. for most shirts (s-l), i'll start the image 4 fingers down from the bottom of the collar- for bigger shirts, i use 5. you may use 3 and 4 depending on how huge your hand is!