Those topping up with formula, how much?

Reg 🇬🇧 👨‍👩‍👧‍👧🤰dd 23/1/19

We are topping up after each bf due to major weight loss., just wondering if anyone else doing this and how much formula is lo taking as a top up each time?

My lo can bf for 30 mins then have 50/60 ml then back to bf x