Just for my own education- where that error would come from? Let's say I take a cup of pool water, put it in some clean bucket and then add 3 of the same cups of distilled water (to eliminate tap water additives), mix it up and then take 10mL of the mix for testing- wouldn't this dilution bring concentration of everything I measure down x4 times exactly?... It's also fair to mention the amount of error with a 3:1 dilution. ...
Tap water can bring FC error as I read some areas have up to 1 ppm which would multiply by 4 at the end. My own tap water doesn't have any < 0.2 ppm so I used this technique when I was coming down from 300ppm CYA.