You really really really don't need to over think this.
Here is what I do.
For a 20 gallon tank I do 70% water change (approximately) once a week. Package directions is to add 1 ring (in the cap) of prime per 10 gallons. I add roughly 20 gallons worth. I do not measure precisely. I eye ball it. If too much or too little goes in I don't think or worry about it at all.
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