For non-liquid weight, I believe it is 28.3495.... grams per ounce. I round up to 28.35.
I divide 454 by 16 to get 28.375.
Either way I use a stick, some string and some coins to weigh my weed and don't plan on buying any more tight ounces now I can grow my own. (UK coins are 1p is an 1/8 and 2p a 1/4 oz.....nice and handy )