Yeah you've done the right thing not using tank water in the dc, it can show misleading results due to various acids present in the tank water. 4dkh water is correct with a few drops of low range ph reagent mixed in too.....the dc is basically a ph test kit which reacts to changes in acidity caused by co2 gas exchange. You can add as much co2 as you like whilst there is no fauna in the tank and the plants will thank you for it as they already seem to be
The dc is a good guide, but that is all it is, once the livestock go in its all about balancing co2, light and distribution.
You are doing everything right at the minute so don't worry and carry on.....it's nice to be doing it correct
Cheerios,
Ady