I rarely calculate the amount of water I change even if I use buckets! It depends which conditioner I use. When I use the Easy Life, I add a full dosage (meaning, the whole tank volume) and if I use the Sera one, I dose each bucket separately. It really doesn't have to be that accurate since a small amount of extra water conditioner won't be bad! My kinds are both the type that you can add more of in case you have new or sick fish
Sounds like Dragoniis method is really good!