Years ago tried making some out of venison and 25%pork butt. Not very good and was told because way to lean. This year while grinder is out for venison thinking would try again using 100% pork butt. If I buy those vacuumed sealed ones like at Sams with about 3/8" thick fat cap how do you know how much additional fat to add? Was told commercial brats where around 40 to 50% fat.