how do you measure amount of breastmilk?

So I am confused about how to measure amount of breastmilk using the Lansinoh bags. My sister in law said that I get the air out and then measure the ounces, but that gives me an amount much different than what my pump bottles say. For instance, if it shows I pumped 2 ounces based on the bottle, I'll put it into the Lansinoh bag and just as is with the air still there it'll say 2 oz but once I take the air out it goes to 3.5. I'm really trying to figure out about how much my baby is needing per feeding but I just don't know which measurement to trust.