First, don't follow the directions that come with the Red Sea indicator. Instead, fill it with 4 dKH distilled or deionized water and a couple or three drops of pH reagent (the type that is yellow at low pH and blue at high pH.) Now, when the color is green you have about 30 ppm of CO2 in the tank. If it is blue you have a lot less than that, and if it is bright yellow you have a lot more than that. The goal to shoot for is 30 ppm, so yellow means you have too much CO2 and blue means you have too little CO2.
If your tank contains only small fish, around an inch or so long, and you have good circulation and surface movement, you might do fine with the yellow drop checker. Tom's testing in his big tank showed surprisingly large variations in the concentration at various areas of the tank, including some areas well above 30ppm, but no distressed fish. That tank has a heavy ripple at the surface and very good water circulation.