You have a healthy sized baby, so the books may not be taking that into account?
I think it's important to remember that all babies are different and only he can tel you when he is hungry or not.
I expressed everything I could, every time he fed, then gave it him at next feed. Anything he didn't drink, I froze. BM freezes for 6 months, and my doula told me can be kept frozen for 1 year, so it is well worth doing. When I wanted to give up breast feeding (6 months in) I continued to bottle feed him with milk from the freezer, and expressed milk until he was 10 mo. At 10mo I stopped expressing, but continued to feed him milk from freezer until it ran out at 14mo.
It was wonderful to have the freedom of bottle feeding in those later months, but know that he was still recieving nutritious breastmilk.
If you are going to do this, buy some bags and remember to label them with the date, that way you make sure you use the oldest milk first. You can use ice cube trays to freeze cubes and put them into the bags, then just grab a couple of cubes to stick in the bottle and defrost.
Good luck, and remember you are doing an amazing thing for your little one.