No, the fat won't full separate from the milk just by gravity. Based on taste I've always thought it comes out to be about 2%ish. And doing the math, I usually skim about 1/2 gallon of cream from a 4 gallon pot, and from that 1/2 gallon churn about 1 pound of butter. Subtracting that amount of fat from the remaining milk, it comes out in my case to about 2.2 to 2.5% fat, after starting with 5% fat (which we learned from having a milk test).