Personally, I don't normally use domestic fat. If there's not enough fat on the hog, I just smoke it or turn it into ground meat. When I do get a fat hog I use the 'hard' fat off of the back and sides and mix it in with the rest of the meat to make sausage. That said, I think the general rule is that you need about 25-30% fat to make sausage. 20% is the bare minimum. You may, of course, use more if that's to your liking.
You can never have too much ammo — unless you're swimming.