For most accurate values you should get your RO water tested. If you have an RO filter it may work well, and may not work so well, so it's best to get it tested (home-scale RO filters are not as good as commercial units). Short of that you could use commonly cited values for RO water, like found in some brew water calculators (like Bru'n Water for example).
I have our RO water tested and adjust values for our water under the ingredients tool.