My ball park guess for using yogurt that is fully coagulated ( 4.5 or lower ph ). Or like soft cooked egg whites in texture. Falls apart easy enough when stirred
1/4 cup or so less per 1 gallon
Also if you want to get good at this with yogurt ,( a highly variable starter ) you will have to start keeping records
By the way is this store bought yogurt or home Cultured ?