You need to test your water for the KH (not GH) hardness. Once you know your water's KH you can set the pH controller to hold a pH level that corresponds with the proper Co2 level given in the graph above.
The numbers in the center of the graph are for Co2 in ppm. The green shaded boxes are the "goldilocks zone" of enough Co2 to keep plants happy, but not so much you're going to gas your fish. 15-30ppm seems to be the general rule of thumb.
(rough example - your numbers will be different most likely)
So you test your water's KH and get "6". So you go to the graph, find "6" on the KH scale and follow the boxes over to the green boxes and see that 18ppm Co2 will give you a pH of 7.0. Now you go to your pH controller that will turn the Co2 solenoid on/off and set it it hold 7.0pH. Thats it.
Beyond that you can start lowering pH very slowly to see if the plants like even more Co2 and if the fish can tolerate it. Most tanks seem to end up somewhere between 6.5 and 6.8 pH depending on KH hardness.
Correct, even drop checkers are only going to get you a rough estimate. the key is start on what you know to be the safe side of that estimate, then slowly start increasing Co2 concentrations while carefully watching plants and fish. And by slowly I mean over days and weeks of very minor adjustments at a time.
If you have a cheaper single stage regulator you can get an "end of bottle dump". Basically what happens is the pressure falls too low in the Co2 bottle for the regulator to regulate and it opens the valve fully and your bubble count goes from 1-2 per second to full flow. This will suffocate the fish in short order.
The fix is to buy a quality two stage regulator which wont do that. They cost more, but worth it in the long run.
Also, if you are not reading your KH right or your pH probe is out of calibration, then you could overdose Co2. that's why you always want to be testing water, calibrating probes and making small changes when you do change something.