I think it really depends on the baby. My daughter was born Nov 25th and I mix 3oz of formula, 1oz breast milk and 0.5 oz water (water added per pediatrician due to infrequent bowel movement) every 3 to 4 hours.
I kind of had the same problem..my baby was born Nov 30th I wasn't producing enough breastmilk to keep him full so now I feed from each boob and then give him a 5oz bottle of formula after every 3-4hrs