I have only just started introducing a little bit of milk, and I am doing it only for one of his day-time bottles, which is his small late-arvo "snack"" bottle. For his "major" bottles, I will continue with formula for a while until I feel he is a little further along in his solids journey. With his late arvo bottle, I put in 2/3 formula and 1/3 milk- for my formula, that is 50ml water and 1 scoop of formula, and 25ml water (its only a little bottle!).

HTH!