Most pH test kits have different colors that are 0.2 units apart. Maybe I'm partially color blind, but I find it difficult to tell two adjacent colors apart most of the time. That means that you really don't have any idea what the real pH is. You could be off by 0.2 or 0.4 units, even if your kit is perfectly calibrated, the test solutions are new, the comparison charts aren't faded, etc. I finally gave up and went with a pH meter. This might not be within your budget, but at the very least, get a better test kit.
Or, don't freak out too much about it. Turn the CO2 up gradually and watch the fish carefully. If they show signs of distress, back off a little on the bubble count. I think a KH of 3.5 should be fine if you don't ignore the tank for long periods of time. Once you get the parameters 'just right' it will still take a couple of weeks for new growth to overshadow what was formerly covered in algae.