I really like using a "drop checker" to measure CO2, but probably the best way to do it easily is to measure the pH of the tank water after the CO2 has been shut off for 12 hours or more. Then, turn on the CO2, and wait until almost when you are shutting if off again, and measure the pH again. The amount of CO2 you have in the water at that time is about 3 times 10 raised to the drop in pH power. For example, if the pH after the CO2 is off for a long time is 7.6, and the pH after the CO2 has been on all day is 6.8, you have about 3 times 10 raised to the 0.8 power(which is 6.3) or about 19 ppm of CO2. A pretty good CO2 concentration to try to get is 30 ppm, which corresponds to a 1.0 drop in pH.
With this method, it doesn't matter what the KH is.