Glad to help. Good choice with the Betta and CO2 ... gives you a little more wiggle room. Keep in mind for such a small aquarium you need very fine adjustments.
What I mean about the dropchecker lag is that if you reach optimal values (~30ppm CO2) at 12:00, your drop checker will turn the right green color for 30ppm at 1 or 2:00.
The lag you observed is independent of the measuring method. It takes time for the CO2 to build up to a certain concentration and it takes time for CO2 levels to drop down from a certain concentration. This is why most start adding CO2 an hour before the lights are turned on. This depends on the injection method, surface aggitation, surface to volume ratio, aquarium size etc. To give an example, if i take a cup of water from my aquarium and leave it in the room, it takes about 20h to get a stable pH of 7.5 (degassed pH). The water in my aquarium never goes over 7pH ( even with CO2 injection stopped, it is still there ). When I start adding CO2 the pH drops to ~6.4. I double check my pH measurements but even if they are off, the drop checker tells the same story, just with a 2h delay.
Once you reach your target CO2 you can turn it off. For how long that is a more complicated question ... The dropchecker will change colors too slow... but adjustments can be done over some days. Say you want lime green. Once the dropchecker reaches that point turn the CO2 off and record hour H1 (for Ex =12:00). When the dropchecker starts to be dark green record hour H2 (=16:00). Now to adjust your schedule ... turn off your CO2 at H1-2 (10:00), turn on at H2-1:30 (14:30)... and repeat until you have a consistent color from lights on to off. You can see why a pH measurements is more accurate and why most people do not bother turning it off and on during lights on period.
The most relevant thing for this discussion is that CO2 will lower the pH but not the KH. In theory, if you have a stable, known carbonate hardness (~KH) you can approximate the concentration of CO2 just by looking at the pH. So you will have a good/high CO2 (~40ppm) at pH 7 when KH is 12, but the same ppm CO2 will give you a pH of 6.4 at KH 3. Here is a detailed table about it http://www.barrreport.com/forum/barr...o2-ph-kh-table