Yea it's a head scratcher innit? LOL.... I've been using the 160g/500ml and dosing at about 1 drop per gallon, but I must admit I tend to be pretty liberal with it. When I do a water change and I change out 20 gallons I will use about 10ml of this mix. Do I need to? I don't think so. The sniff test on my gallon jugs of top off water suggests that the chlorine is gone at 1 drop per, but I err on safety. At $3.25 a lb I'm not worried.
As far as ODing this stuff, I think it's pretty universally thought that it's difficult to do. Surplus just oxidizes out of solution and leaves salt.
HTH and Greetings back to ya!