Mix Thawed breastmilk with Formula

Hi mummies! May I know if any of you have tried mixing thawed breastmilk in the same bottle with formula before? I used to freeze 180ml of breastmilk in each milk storage bag, but now my baby’s intake per feed is arnd 200-210ml so need to top-up the balance with formula but I’m not sure how to do it! My girl is 14months old!