yes. its fine. you only need to dose for the amount of new water added but the companies want you to use more so they say to dose for the full volume.
when you add dechlor to the existing water, there shouldn't be any free chlorine in it to react (evaporates as it is a gas at standard temp/pressure), nor chloramines (which last longer) because it was all neutralized last time you did a water change (dose for full volume when filling up tank from empy of course).
then, as your new water hits the old water + dechlor, it mixes and reacts as you go.
now, if you have an ammo/nitrite spike and you are using something like Prime that reduces it, then dose for full volume.
but if you are only use a straight dechlor, just add for the volume of new water.
you can overdose, it won't really hurt anything, but sometimes you can get a bit of cloudy water if you add to much (probably a precipitate reaction)
hope this helps