just to reiterate what Ed quite rightly said..
the ph in a tank can be altered by a large number of factors other than co2 content, this is why we started to use a drop checker in the first place, containing a lab tested KH solution, and not tank water. 4dKh was decided upon (by quite a few of us on the barr report) because it went green at the desired 30ppm (+- 10ppm).
Arana 🙂 yeah 30ppm is fine, as would be higher than that to be honest.. just make sure you have adequate surface agitation and dont add co2 when the lights are off. (although there are mixed theories still about that). Gas damage to fish isnt caused by co2, its caused by LACK of O2, adding co2 doesnt replace O2.
basically. test kits are far too inaccurate measure co2 with any certainty, even if we calibrate them to the best of our home based equipment. This is where the Estimative Index really is worth its weight.. once your sure your ferts are in order, you can throw every test kit away, and start thinking about scaping, which is why we do this in the first place 🙂