Id be careful of probes and computers, they're just not accurate enough to control things properly.. the only way to be sure is with a drop checker. A PH probe measures the PH of the tank water, which is widely known as highly inaccurate way of doing it.. far too many other factors besides CO2 can alter the PH of the water.. it will lead you to have problems and highly unstable CO2.
Drop checker using reference solution is the only sure way of doing it.
i understand that ph probes measure ph levels, but surly this is used as a guide and if used in conjunction with a calibrated drop checker it will give you a suitable tank rating/ph level based on a 30ppm drop checker reading, so i feel that a +/- 0.05 reading that my probe will control via my solinoind will not only save my fish but also my co2 supply as opposed to whole heartily relying on a drop checker that is already 2 hours out of date when you read it...!!!!!
i have to disagree with your point about co2 stability and and your reliance on solely a drop checker,,, that I'm sure will read differently at different tank locations....