With low KH you get low pH for any specific ppm of CO2. But, I know of no way to say what that pH will be when the tank water contains tannins, phosphates, and other compounds that affect the pH. You really need to carefully adjust the bubble rate, while watching the fish and plants, until you get it to the optimum value. Just increase the bubble rate slightly, watch the fish for signs of distress, specifically gathering at a top corner seeming to be gulping air, or laying on the substrate, or fading out in color. Also, watch the plants for 2-3 days to see if they seem to do better, or pearl. If the plants do better, and the fish aren't distressed, repeat this, continuing until you see no improvement in the plants over a few days, and no fish distress. You will then be at the best CO2 concentration for the tank as it is, with the plant load it has, with the plant mass it has. And, the bubble rate you end up with is going to be lower if you don't have the water well oxygenated, and keep everything clean, plus have very good in-tank water circulation.
If you just use low to low medium light that process isn't that important. Then you can use a drop checker to get somewhere near the optimum, but probably less than optimum, and just use that, since the plants probably won't need any more than that.