How do you know how much to give- since the range is so big

My baby is about 3 weeks old and eats anywhere between 90-160ml each feeding

Oh ok.. My little one has been drinking 3 oz every 3 hours but at times he just cries his lungs out even after breastfeeding him for an hour in addition to the 3 oz if firnula

My little one is eating around two or three ounces every two or three hours, but she has days where she just kinda changes up her entire feeding schedule.