80 / 20, for example
How is the ratio of beef to fat determined? Do they take 12.8 oz of beef (which already has some fat in it) and add 3.2 oz of fat to it?
Essentially, but in much larger proportions. Take 800 lbs of lean ground beef, add 200 lbs of fat to it, process, pack and sell. Chuck is about 20% fat right out of the box; not much fat is added or taken away when processed.
If you're asking about making your own, you'll want extra lean beef, well trimmed shoulder or top round, preferably, and add 20% fat. Or use chuck and brisket and skip the added fat. 80/20 is what I like for burgers or meatballs; other than that, I use a coarse grind 90/10 for chili.