If I have converted right then Your KH is 3 and GH is 1 ..
Take a look at this table it is very informative and will enable you to work out the correct PH you need to hit for maximum benefit
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm
from your current readings 6.8 is right for the amount of Kh you have ..you could take it down to 6.6 ...