Thanks for the answer, I feel a bit better now!
I supplemented with formula just 3 times, during 3 consecutive days in the evening, when my breasts really felt empty, and my baby still looked hungry after having had both breasts.
It's been a week since the last bottle of formula and she's gaining weight just fine. However, today my breasts feel empty again
I will do as you said, keep offering the previous breast if I think she's still hungry after both breasts!
Thank you very much for the advice