I really believe that those crictiria for baby weight aren't reasonnable and pressure mommy to use formula when this isn't necessary for many case. I felt into that trap and my daughter ended up having constipation. Once i discover that all i needed is to nurse more often (me i was pumping), the milk went up and i didn't need to use formula ever again. I had so much milk that i had to freeze it. I thought also, that i was one of those mom not being able to produce milk. There are natural tea that they sell to increase the nursing milk. Nurse more often. Baby goes to many phase of bursting hunger.
The best food for baby up to their teeth start to growth, is the natural milk only and NOTHING ELSE. Nurse more often, if you can invest, buy an automatic pump, so you could pump some extra milk when your baby doesn't want to nurse, and you could use that milk for the night. I love my pump and i still use it even though my daughter is 3 now. Very handy that pump.
Best wishes!
:)