I have an API gh/kh test kit as well that I was having problems with, however I think that I found out what the problem was. When trying to determine my kh, I would add the very first drop of solution and the test tube would still look clear. I could keep adding more and more drops, but it stayed pretty much clear until it suddenly got very very dark orange/yellow. The problem was (at least I think) that my kh is >1. I knew that my kh would be low, but I wasn't exactly sure how low. If you have a water sample with a kh higher than 1 or 2 or so, it is fairly easy to notice the sudden change from blue to yellow.
When I mixed up a solution to put in my drop checker, (if you do not have a drop checker I would highly recommend that you get one, and it doesn't have to be an expensive fancy drop checker) I got some RO/distilled water and poured a few cups or so into a clean container. I then used a toothpick to add baking soda to the water. Add a little baking soda, test the kh of the water. If the kh is less than 4, add a little more baking soda with the toothpick, if it is higher than 4, add a little more water. Continue this process until you have water with a kh of about 4. It does not take very much baking soda at all.
When I tested my mixed water for the last time, the first drop turned it light blue, the second a more solid blue, the third a very solid blue, and the fourth turned it a dark yellow/light green. A fifth drop turned it very light yellow. Therefore, I determined that my kh is between 4 and 5 degrees.
With my kh being between 4 and 5 degrees, according to your CO2 chart, I should have between 30-37ppm of CO2 when my drop checker is green. (If I remember right, the indicator solution used in most drop checkers is blue at a ph of 7.2, green at 6.6, and yellow at 6.0) Therefore, my CO2 should be just at or slightly above "optimum" levels. Optimum levels seem to differ person to person, but it sounds like generally 30-35ppm is a good number to shoot for.
It takes a little bit of work to get distilled water with a kh of 4, but it helps give you a more accurate idea of how much CO2 you have in your tank. It's not a perfect test/indicator, but it is still fairly accurate. Sorry for the long reply, but I hope this helps.