If you shut down your system and remove 50% of the water (whether through buckets, siphon, over flow, etc.) you have in theory removed 50% of the nitrates. On the other hand, if the system is not shut down and you add 50% more water to the tank and let the surplus over flow from the system (from the sump, a siphon, etc.) you are very unlikely to remove 50% of the nitrates.
The new water is going to mix with the old water and you're going to dump some of the new water in your water change. How much depends on your particular setup, but it could be substantial.
Water is fairly cheap, but I like to know that my 50% WC actually removed 50%.
Keep in mind that some of the dechlorinator will get dumped as well, so you'd want to take that into account.