A DC (Drop Checker) with a pH meter/pen or test papers, I trust the DC colour change as I have good flow in my tanks, if the flow is inadequate or set up incorrectly the DC colour change can vary in different parts of tank. Having good flow/turnover in tank resolves many CO2 related issues. Flow is King of the high tech tank. When my 50l gets overgrow the flow is hindered and my DC colour change becomes unreliable and CO2 levels fluctuate. A good trim resolves the flow and CO2 levels become good again
If your DC is green after two hours and the pH remains stable from lights on till CO2 off job done. However what is stable ? I aim for less than 0.1pH change from lights on till CO2 off - which is hard to achieve with BPS adjustments and takes many days/weeks to get it right.
Once you have a stable pH from lights on till CO2 off do not adjust the injection rate (BPS) again, which is a classic mistake of many folk new to CO2. The pre CO2 on time is the time it takes to reach your target/stable pH - if it takes 2 hrs 43mins that's how long it takes and adjusting the injection rate to shorten this time is a NO-NO.
There are ways of having better/faster control of CO2 injection, however with a single solenoid and 'normal' timers it isn't possible/advised
Unless you have very soft water or using RO/rain water, your dGH and dKH should be fine and the actual pH of your tank is irrelevant unless your livestock needs a specific pH range.
A 1.0 pH drop from CO2 on till lights on is the aim and will achieve 30ppm CO2 - Caution however as your tank may not be fully degassed from the day before. I take a glass of tank water and leave it 24hrs and use that as the base line pH off tank - eg pH off glass of water after 24hrs is 7.6pH then my target pH will be 6.6pH. It doesn't have to be 6.6pH you may be happy at 6.7 or 6.5pH.
Having a stable pH with good flow from light on till CO2 off is far more important/critical than the pH drop/[CO2]