It does makes sense... but you start by saying the end result, which makes it more confusing. So you have 5ml of water reading something above 40mg/l. You add 5ml of de-ionized water, meaning you now have the same amount of NO3 in twice the water volume (10ml). If you can read something bellow 40mg/l, let's say 25mg/l, in 10ml of water, it means you originally had 50mg/l of NO3 (=25x2) in 5ml of water. If, on the other hand you still read 40mg/l, you have to repeat the process, adjusting the math.