Serious Eats - seriouseats.com

Congrats on your first try at yogurt. But you're doing way too much work.
All you need to do is bring a half gallon of milk to 180 degrees F in the microwave in a pyrex container, or on the stove with a nonstick pot. You'll either need to continuously stir to prevent the milk proteins from sticking to the bottom or not worry about it and pour off the milk into another container after it reaches 180 F, leaving the stuck on the bottom part behind (hence the nonstick pot). Let the milk cool to 105 F and then whisk in 1 tablespoon only of yogurt. Just like using less yeast and fermenting longer results in more flavorful bread dough, less yogurt and a longer ferment results in better flavor. I've used Fage but personally like the tartness of good old non flavored Dannon. Leave your pot of milk in the oven overnight, covered. At least 10 hours for tartness. Or, put it in your parked car with the windows rolled up.