Is your co2 system working with the ph controller ? If it is your controller will keep the PH relatively stable if you add your RO water into the tank slowly and let the Ph stabilize. The nice thing about the controller is that it gives you instant Ph readings. A ph variation of .1-.2 shouldn't make any difference as its getting mixed in with the tank water. I think you can add the RO water over the period of 4-6 hours with no ph shock with your controller regulating co2 to adjust for ph.
FILSTAR Pimp #2
75 gal heavily planted,50/50 Black beauty,Eco-complete substrate, Pressurised CO2 with solenoid, ph controller, AB Reactor 1000, 330 watts 9325K GE PC lighting,Ehiem Liquidoser, 2-Filstar xp3 canisters.