Dosing auto top off vs dosing tank directly

[Jasoned]

So being a new reefer who is just starting up my question is, why don't people dose their auto top off water vs dosing directly to the tank? I think it would be cheaper to figure how much water gets topped off per day and then add the desired amount per amount of ato water you have. Then you'd basically dose each time you refill your auto top off tank.

Maybe I am missing something but doesn;t that sound doable? Or maybe ato varies per day?

Thoughts?

Jason