If a 158 grain LSWC bullet is ordered, what should the finished +/- weight of that product be

There is no "industry standard". The +/- factor depends on the mfg's quality control of their product.

A lot of molds are made to cast a bullet of pure lead at a given weight. If something is added to the lead to harden the bullet it will weigh less. You 158 grain bullet may only weigh 156 grains or less depending on what hardener. If they go from 156 to 160 its just sloppy work!

actually most [good] molds pour heavier than their stated weight.

if all the bullets are within a 3% weight range you got good ones.
if they are within 5% you will shake more than they will vary in group size.