^^ Baby gets more milk out than the pump does :)
I did after two weeks of having my son. I would breastfeed him for about 30 to 45 mins each time at night, but I really didn't know how much he was drinking. So I pumped for 30 to 45 mins and got 2 oz out of each breast, so it kinda gave me an idea of how much of breast milk. so I would just do 3 oz of formula since it makes them more full.
First of all don't listen to anyone trying to preach to you.
Do what you want! And it's fine to breast feed and formula feed at the same time, what about all these women who done produce enough milk...their babies need formula top ups.
On the formula tin it should tell you how much he requires for his age Hun
Breast milk is always the right temperature, it's made for your baby, their poops should smell less, baby will be healthier and happier (because your body made baby and the right milk for that specific baby), breast feeding gets you back to pre pregnancy weight faster and a thousand other things!
I don't believe that using both is the best option? That's my personal opinion based on the information I have read and heard. Someone else might disagree with me but if your gonna breast feed... Do it. The benefits for baby are endless and so are the benefits for you