Formula question

So I use formula and BF. And it’s easy to know how much he eats with the breastmilk, like 2.5-3oz every 2 hours bc it doesn’t rise like formula. However, with the formula I do the water at 4oz, then the two scoops but it rises to like 5oz? So if it goes from the 5oz to 3oz, did he technically eat 2oz??? Even tho some of its foam from shaking it? Or should I count it as a 4oz bottle since it was 4oz of water but rises so he’d have eaten 1 ounce?? Sorry if this seems dumb lol.