i always just fed ours when they were hungry (both of ours had acid reflux so we did it because they typically lost a lot of what they eat when they got sick) and made a specific amount that they "typically" ate at that point in their life. if they wanted more i would make half of that amount and give it to them (trying not to waste) and they were typically full. if not, i'd make a little more but was always careful not to over feed because of the A.R. after a while, i noticed a schedule of when they were typically hungrier, and when they typically ate less as well as when they wanted to eat. both my boys did their own schedule, i just had to figure out what they wanted it to be. with us, this worked well, we had them sleeping through the night on 99% of nights within a week or two and they were happy as could be.
don't worry, though, it'll make sense soon just don't worry about the small stuff! do what you think is right for you and your baby.