you don't have to convert, skip the degrees part. Type in your pH, then skip the degrees box, type in your KH reading ppm in the box to the right. Then when you click on the degrees box, your reading of both your co2 concentration and KH reading converted to degrees should show, pretty neat, took me a while to fig. out.
but i'm running 16.6 w/ DIY co2, i have the stuff for a co 2 tank, just can't find a co2 tank that is ridiculously priced, any suggestions?