Hi. Drop checker is the easiest way. You can make it complicated if you want, but I just aim for 1-2 bps and then see how things are going. If all is well I bump it up a little and watch over the next few hours. If you see fish gasping, or plants pearling super hard, dial it back some.
As for your pH, you really need to consider using a softer water source. Plants uptake nutrients the best when the pH is in the mid 6s to around 7, so getting in that area is your best bet. It's a lot easier to make water hard than vice versa.
Yes RO is an option, or using some distilled water. Or rainwater. Just depends on how creative you want to get.
Having pressurized co2 without a drop checker is like Harold Melvin without The Bluenotes....you'll never go platinum.
Try EI dosing:
http://www.aquariumfertilizer.com for macro ferts