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Triton, the 20ml was just mentioned as an example.
Here is more info and what I do:
5 Liters over 2 weeks then:
5000ml/14(days)/12(times a day) = 29.8ml every dose.
So to sum up, dosing 29.8ml for 12 x a day should make your 5L bottle last for 2 weeks or 14 days.
The bottle size and how long you want it to last is entirely up to you, but I found the 5L over 2 weeks works great for me.
Hope this is a little clearer.