Well, I finally got a chance to take a good look at my dechlor today. I do have the Seachem Prime product. And, sure enough, as Rex stated, it said to use enough for the whole tank IF you dose straight into the tank. It was down in the instructions, and I likely glossed over that part after reading the first half about dosing per the volume of the water change.
So, at of today, I'm now dosing based on the tank volume since I use a Python and just add water and dechlor straight into the tank.
Thanks for the comments, all!!