How to determine baby’s milk intake if theres leftover.

Hi mummies i have a very important question about baby’s formula milk and i am confused. Applies only for formula milk. When baby’s intake is 150ml, we pour water 150ml then put the formula powder, itll become like 160/170ml right. If baby end up not finishing certain amount like 40ml, how do i determine the amount he drank? As i am consistently taking down baby’s intake. Is it 150ml - 40ml OR 160/170ml - 40ml? Cause for now i just take it as 150ml - 40ml. I am so confused.