Hi and welcome! I don't know which book you are readng but there are sample routines in the baby whisperer solves all your problems book. the idea is not to feed on a rigid schedule though, and to watch her cues, especially when she is that small. I would definitely feed on demand at 3 weeks old. in terms of amounts, I would also go with what she's having, and make sure to always put a little more in the bottle so that she stops feeding because she's not hungry anymore instead of just because there's no more formula in the bottle.
personnally I let DS sleep but would wake if it got 4h between feedings, to be sure he was getting enough nutrition during the day. of course if something exceptionnal was up and I could see he really needed to sleep I could let him go a little more, but not too much because you want to be sure they are not mixing up day and night.
so you're saying she cries from 7 to 10 only when she has napped well? I don't really have advice for you on that, for us it was the opposite.
maybe you could post on the EASY board to get some help with routines?