Normally, you need to add just enough dechlor to treat the new water you add. For some reason, some conditioners (e.g., Prime) suggest you dose for the volume of the entire tank, not just the new water.
When I use Amquel to treat partial water changes that go straight from the tap to my tank, I dose for the amount of the change, plus a little bit more in case the water utility went crazy with the chloramines.
HTH,
Jim