Your doing it correctly, my tank water is 0.8-1.0 KH and at 1.2 ph drop, the checker is still pretty dark green. Also you are correct to go very slow, the lower the ph goes a 0.1 increment will show a larger and larger co2 input. It will turn from okay to too much very quickly, once your checker is light green to slightly yellow your close enough as long as fish are acting normal. Every tank is different and I wouldn't worry too much about your final ph value. Even after dialing it in the co2 will fluctuate with many factors, surface agitation being a main cause. Water evaporates, filter output is closer to the surface and increases gas exchange, co2 off gassing increases while input stays the same.
Add enough co2 to make the plants happy but not as much to stress any fish. Not many people actually know ppm of co2 is in their water, it's too expensive to test for.
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