Formula feeding uncertainty! How much are your LO's eating?

I know the general rule is 2.5 ounces per pound in a 24 hour period. My LO is 8 weeks & she is 11 lb 9 oz so she should be getting 28-30 ounces a day. She sleeps through the night so she only has 6 feedings a day which means that in order to get 28 ounces she has to take a 5 oz bottle at some feedings. She's been taking about 24 ounces in a 24 hour period but then for a few days she went up to 28 ounces & was doing great. Now for the past 2 days she's had 24 & 22 ounces & has only taken 2 ounces at some feedings-something she hasn't done since she was 2 weeks old! She's acting like her usual self & sleeping/wetting diapers normally, definitely doesn't seem to be sick or anything, so what's going on? Any ideas? And how much formula are your 8 week olds eating?