34 weeks Feeding Amount

My baby arrived at 34 weeks and 3 days. He is almost a month old. When he left the NICU, he was bottle feeding 45ml of my milk and supplementing it with a little bit of formula for extra calories. I have increased his bottle to 50ml. Should I increase the amount more? I don't want to overfed him since I know his stomach is still really small. How do you determine how much to feed?