I add mine before I do the waterchange. I add it directly to the aquarium. I add it for the volume of replaced or new water. I see, however, I may be doing it wrong... I think I stopped reading 1ml/10gal of NEW WATER... and didn't see the note about dosing directly in the aquarium and using the entire volume. HEhe.. whoops. No fish losses yet!!!
From what i understand If you only dose for the amount replaced, it could be all used up on just nitrates (especially if you dose nitrate) and not totally declorinate the water. Maybe this applies only to prime. Thats why i dose a full amount every-time. I could be wrong, but I'm paranoid. But i don't think it matters when you add it. I just start the water then add it.