How are you adding water back to tank ?
If you are using buckets you can dose for amount and add it to the buckets.
If you use a hose to fill direct to tank then you should according to directions dose for total tank volume ..
Agreed with coralbandit
I use 5 gallon buckets so I use .7 ml for each bucket. If you are filling up with a hose or Python syphon, dose the entire volume of the tank then fill up the water.
I also use 5 gallon buckets, so at two drops per gallon I just put 10 drops a bucket. However, I also have one bottle that isn't a dropper which I use for when I'm cycling or have an issue with my cycle, in that case I just dose the whole tank volume in the first bucket