Basically target these pH levels for the optimum CO2 content (30-35mg/l) in water:

Kh=2 ----- pH should be 6.3
Kh=3 ----- pH should be 6.5
Kh=4 ----- pH should be 6.6
Kh=5 ----- pH should be 6.7
Kh=6 ----- pH should be 6.8

Make sure your pH drop is only due to the addition of CO2, which means no using of peat or any pH down (this product is a con-job if you ask me - just use CO2 just like nature). If you're not getting these pH values for your specific water Kh, you don't have enough CO2 in the water, so increase more bps till you achieve these pH values. Hope tis helps