I make ricotta cheese by heating milk to high heat with buttermilk and sour salt (I think it is CaCl) until it curdles and drain off the whey. The volume reduces by a factor of about 4. This means that, starting with 3.25% (the abomination that passes for “whole milk” in Quebec, it should have a BF content of 13%. But I am sure that some of the fact goes awhey. Many years ago, I watched a cheesemaking factory in Gruyere and it was worth their while to put the whey through a cream separator.
Googling the question just leads me to many sites on how to lose fat by eating dried whey protein, which is no help. Does anyone know even an approximate answer?
Too late to edit but more relevant to the OP, according to this site acid whey has a fat content of .22 g per cup, which is roughly .1%, while sweet whey has .89 g per cup or around .4%.