"FRBSTRD" wrote in message
...
wouldn't know i juts add it till i get a reading of approx 5.ppm with my
test
kit

Errr, yes, but I wanted to calculate how much to add beforehand, so I'm not
wasting my tests and I don't spend ages adding a little more, testing,
adding a little more, etc etc, only to find I've added too much.

Marcus