Im still keen on going with RO water - ie. 50/50 ratio
@dw1305 explanation and example above is excellent - I think there is little point in investing in an RO system, just to mix it 50/50 with hard tap water, and still end up with relatively hard alkaline tank water. you should be aiming for a final carbonate level that results in <2dKH as Darrel suggests, though personally I would aim for less than 1dKH (well in realty I actually target zero dKH in all my tanks simply by just not adding any carbonate sources).
You need to focus on the parameters you have direct control of, such as alkalinity through carbonate additions, and general hardness through Calcium and Magnesium additions, rather than the pH which you have no real direct control over - and most definitely artificially targeting a certain pH through CO2 dosing is really not the way to go about it.