How much yogurt should I use as a starter?

STEP 3: INOCULATE WITH THE STARTER CULTURE. When your milk has reached 115°F, place the plain yogurt (the starter ) in a medium bowl or glass measuring cup, using 2 tablespoons yogurt for ½ gallon milk or ¼ cup yogurt for a gallon of milk. Ladle in roughly 1 cup of the warm milk and whisk to combine.