Harley,
Kent does not treat the same amount of tap water as Seachem Prime, as you suggested in the link below.
http://www.monsterfishkeepers.com/forums/showthread.php?p=3807875
From the Kent website:
Shake well before using. Add 5ml (1 tsp) per 50 gallons of tap or aquarium water. Will remove up to 2 ppm of chlorine/chloramine at this dosage.
From the Seachem website:
Use 1 capful (5 mL) for each 200 L (50 gallons*) of new water. This removes approximately 1 mg/L ammonia, 4 mg/L chloramine, or 5 mg/L chlorine.
For those who don't already know this: 1ppm = 1 mg/l
This is classic example of why one needs to know exactly what they are treating for, before they begin comparing dosage rates, and overall cost.
You can't simply use the "one capful for 50 gallons" for all applications, or you could be using twice as much water conditioner required, or half as much, or anywhere in between. Using your .7ppm example, one capful of either product would indeed be
massive overkill. In your case Kent Pro performs as well as Prime because if you are going by "one capful treats 5o gallons" you are using over
double the amount of Kent water conditioner required.
Using my local water as an example (2ppm chloramine), 1 capful of Kent would indeed treat 50 gallons, but 1 capful of Prime would treat
100 gallons. If both products cost approx the same, guess which one I'm going to be using?
Some of us here treat 100's of gallons a week, some thousands, and over the years these costs add up.