Its not RA that is used in the acidification calculation, its alkalinity. If you use RO water, you can estimate what the alkalinity is since it should fall in a narrow range due to most of the minerals being removed. Alkalinity for RO water should fall in the 5 to 20 ppm (as CaCO3) range. If the RO machine is working very well, use the lower end. If not so good, use the upper end.
The pH assumed for the water to be acidified is not all that important. If you play with the calculator, you'll see that adjusting the alkalinity has a lot more effect on the acid demand than adjusting the starting pH. Of course when playing with the starting pH, I'm talking about a small range that would be typical in drinking water...say 6 to 9 with many supplies coming in around 8. RO water can have a lot of dissolved gases including CO2 since the RO membrane is very permeable for gases. So that means that the pH of RO water can be a little lower than normal. It should be in the 5 to 7 range in most cases. That excess CO2 comes out quickly upon heating.