Formula question

So how do I track how much he eats? He’s almost 3 months and for this long I’ve just been tracking it by how much water I put in the bottle. So if I put 4 oz of water in and eats it all, I count it as 4 oz. But when I add the formula, it gets like, thicker, and it goes up an extra ounce (or more), I don’t really keep track of that part. I’m wondering if I should counting how much he eats after I add the formula? So like, if I put 4 oz and it thickens up to 5 oz, do I count it as 5 oz? Or just 4? I hope this makes sense. Sorry