How long are you at work? 15 oz would be about right if you were separated from baby for about 10 hours per day, if you're following the rule of 1.5 oz of expressed milk per hour of separation. If you're working less than that, it's probable that your daycare provider is overfeeding your baby, and you may need to review guidelines and feeding and soothing techniques with her. (E.g. paced feeding, swapping a pacifier in for the bottle after baby has had 3-4 oz.)
4-5 oz at a time is a very large feeding. Most breastfed babies take more along the lines of 3-4 oz at a time, making up for the smaller size of the average feeding by feeding a little more often.
A lot of moms get into the mindset of avoiding formula at all costs. But IMO, cereal is too high a cost to pay to avoid formula. Yes, breastmilk is the ideal infant food and formula is a distant second- but formula is still closer in composition to breastmilk than anything else you can feed your baby, and less likely to cause digestive issues. If you need to use formula to get you through the workday, don't be ashamed of that. But I suspect that your daycare provider might be overfeeding, so explore that angle first.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"