Not sure if it actually important or not, but if adding directly to tank it says to treat the entire tank volume rather than just the volume you are replacing.
" May be added to aquarium directly, but better if added to new water first. If adding directly to aquarium, base dose on aquarium volume." - from Seachem website
Oh and I should add, I have been doing this for years as well and never had an issue. I slowly fill over a few hours.