The ppm of CO2 at which the drop checker is green is proportional to the KH in degrees. So, a 2.5 dKH solution would be green at about 2.5/4 times 30, or about 20 ppm.
Make sense, and the chart confirms this.
Multiple drop checkers are just overkill, and don't increase the accuracy at all. For one thing each one's reading would have the same inaccuracy due to difficulty in judging the color.
This is the part I thought might improve with different dKH droppers in the tank. I have a very hard time reading the shades of green or blue. Maybe I'm just colorblind and can't tell if it is yellow/green or blue/green.
If I had two data points, I think that might help. For instance, for 30ppm the 2.5 dkH should show a pH 6.4 and the 4.0 dKh should show a pH of 6.6. I think with two reading like this I could narrow down which color(s) on the chart I'm looking at a little better.
Then, there is the fact that the ppm of CO2 varies at different spots in the tank, and it varies a lot.
This also would seem to suggest more data points around the tank would give you a better picture of what your CO2 levels are. Maybe that 30ppm reading you are getting is from the part of the tank with the lowest CO2 levels.
It is best to just rely on a single drop checker as a means of giving you the courage to jack up the bubble rate enough to get reasonably close to 30+ ppm. From there you have to slowly increase the bubble rate, a little each day, until you get some pearling within a couple of hours after the lights come on, or until the fish start to cluster near the top, or show other signs of discomfort with the CO2. That is the only really accurate way to get to the optimum CO2 concentration for any particular tank setup, short of buying a $2000 special CO2 concentration probe and meter.
I'm starting to get a sense that observation might play a bigger role in getting CO2 levels right than actual measurements.
The drop checker on my 46g is showing lime green, so I believe my pH is 6.4, maybe even a bit lower. This is with a 2.5 dKH solution, so I should be at around 30ppm, +/- more ppm than I care to acknowledge. I see some pearling on the bottom of some java ferns, but this may just be left over from last night's water change. Fish seem happy.
I'm using DIY CO2, with 4 two liter bottles, one new bottle rotated in each week.