My ds was 90th percentile at birth and down to 8th by about 5 months. He fed well, met milestones, was bright and alert and happy etc and when I took him to the health visitor I was lucky enough to get a guy who looked at him, said "he looks pretty well to me, don't worry about it" and we carried on as we were. We had him weighed more often to keep an eye on things, but other than that we didn't change a thing. He did really well, thrived, and stayed pretty tiny until he was about 18 months when he started to climb back up the charts again.
It sounds to me like your dd is doing fine and in your position I would be waiting another month before jumping to formula (unless you want to supplement). I think that if she was really not getting enough from you then she would spend all her time at the breast sucking to try to get milk out, but you say that she feeds greedily to start with and then stop - that sounds to me like she is full. I can totally understand why you don't want to cut short feeds, but if you don't think that she is actually feeding then it is okay to stop her unless you want to give her the comfort.