Noddy, I remember a conversation on another forum and a Seachem rep said they changed the directions to make it easier for aquarists to dose and there was a FAQ section on their forum that discussed it a bit more in length.
Ideally you would need to know how much chlorine or chloramine was in your tap water sample at the time you are doing a water change in order to dose correctly. Some people will do a chlorine test or an ammonia test of their tap water prior to doing a water change and then dose appropriately. I think most people just dose generally based on the instructions without taken their tap water disinfectant levels into consideration because it's just easier.
I think the only time there is a concern is if you are under dosing the product because you become complacent with your normal tap water disinfection levels and there is either a sudden change due to a water authority issue such as a water main break, plant maintenance issue, switch in disinfection products or the time of year they may adjust disinfection levels.
There is a topic over on MFK that goes into great detail on the subject but most of the info is in the 1st post regarding dosage but there may be some tidbits later on if you ignore some of the extraneous comments. Here is the link
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