Here is a screenshot from the web interface of the net module. In this graph I'm logging the PH probe value in 15 minute intervals.
You can see the repeated rise in PH at night when the CO2 is shut off and the drop in PH when the CO2 is on. I did some work on the tank and shut stuff off on the 27th hence the gap in the graph data. Late on the 28th, I calibrated the PH probe which involves using a calibration solution of 10PH and hence the giant spike. On the 29th, I measured my KH to be 8 via an API drop based test. My PH wasn't low enough to get the target dissolved CO2 level I was aiming for so I cranked up the CO2 from 2 bubbles per second to 3.5 bubbles per second. The result of the increased CO2 is a drop in PH to 7.04 during the day (ph values are on the right side of the graph).
At a KH of 8, and a PH of 7 we I can use this chart to determine the dissolved CO2 level of approximately 24 parts per million.
Thank you digital aquatics. I was finally able to dial in my CO2 levels with some level of accuracy. The $3 drop checker is OK for guess and check, but no where near as accurate as this method.