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Don't use BPM to gauge CO2.

Take a sample glass of water from the tank about 2-3 hours after the CO2 comes on (fully gassed water) and measure the pH. Then, let it sit out, 2-3 days, to fully de-gas (overnight if an aerator is used). Once degassed, measure the sample water pH. The pH should be higher than the initial pH reading. In order to achieve what is considered to be an optimal level of 30ppm CO2, the pH in the fully gassed tank should be, at least, one full point lower than the pH reading of the fully de-gassed sample water. Raise the CO2 until the tank pH is one-point lower than the degassed sample, but slowly to allow your fish to adapt (may take a week or more). Note: it is important that the KH reading for both sample points be the same, which should be the case if the directions are followed.

Once you have the pH dialed-in correctly, your drop checker should show the green color, but remember that drop checkers reflect the CO2 readings of 2-3 hours ago.

I would also consider starting your CO2 as soon as ambient light enters the tank. That is when photosynthesis will begin.
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