Babys growth dropped in percentile since birth

Hi all

My little one wss born at 3.6kg (7.9lb) at the 78tb percentile She lost about 8% of body weight in the first few days. She took a while to regain this (about 3.5 weeks) so we've been doing formula top ups along with BF. She now is 6 weeks at 4.2kg (9.2lb) and is on the 30th centile. So shes gaining about 200g a week but still small. Shes otherwise a happy baba. J1ust concerned that she's dropped from 78th at birth to 30th centile... has anyone had any experience with this?