Feeding

Hi Mummies! My LO is 42 days old now. I am doing mixed feeding as my supply is low. I DL and give expressed breastmilk during the day and DL, formula feed at night. My LO only takes in about 60mls each feed if I attempt to give anymore she gags hence I dont force feed. I am worried she might be drinking too little.. However there are 8-10 wet nappies each day. Is it okay to let her continue drinking 60mls? I dont want to be underfeeding her.. How can I slowly increase the quantity. Please advise. TIA!!