Most people that use pH controllers to set their CO2 levels end up using them, to kill their plants and grow algae. Either they under dose CO2 and plants suffer and algae moves in or over dose CO2 and fish die.
Your tank waters pH with no CO2, is related to your water hardness, which will change due to water changes, rocks dissolving and hardness being naturally depleted.
Your waters pH with no CO2 is also related to natural acids and/or alkalis that form in your tank. Rotting fish/plants release ammonia raising the pH.
So your pH with no CO2 is likely to be variable on a day to day basis.
Generally injecting 30ppm CO2 into buffered (ie has some dKH above say 4dKH) will drop the pH by 1 unit, regardless of your waters initial pH. Unfortunately pH meters read absolute pH not relative pH (ie change) so can't really be safely used.
Much much better is to use a drop checker. The fluid in the drop checker is isolated from the tank water, this will correctly (albeit with 1/2 hour delay) read the ppm CO2 in tank water.
You can also use a pH pen to measure tank water pH before CO2 is on and fiddle with the CO2 injection rate until you can reliably achieve 1pH drop at lights on.
Also some people use CO2 controllers as a fail safe, set the pH to very low ie very high level of CO2 to turn off CO2 if you are over dosing. Not sure why, as if CO2 is that high all your fish will be dead anyway, so what was the point, just ensure your CO2 injection method is correct and stable in the first place.