A gain is a gain and normal babies DO gain sporatically; small some weeks, bigger others which is why it is better to look at the average over a longer period of time. If you feed, you will make milk so might be worth cutting out the formula and just feeding to an increased demand to get things back on track. Supply is built around demand so as soon as top-ups are introduced it can actually make things worse as there becomes less demand from the breast, the better thing to do is feed often and just follow your baby. I definitely wouldn't be considering top-ups as long as your baby is gaining, however small.