Calculating homemade yogurt's fat
This is the correct calculation. There is nothing going on in the yogurt to change its fat content. (You can't use a similarly easy calculation for carbohydrates, as the culture consumes some of them). 

Your math is correct. Is it "extremely fat"? In yogurt-terms, not at all. I have regularly seen 10% yogurt from cow's milk; yogurts from other species can easily be fatter. So, compared to general yogurt numbers, it's just a normal yogurt.