Don't try and use calcium carbonate in your brewing water. More than likely, the tap water already has high alkalinity (Colorado River water). Dilution with RO water is a good alternative for creating an acceptable brewing water. Since you will likely want elevated sulfate in your brewing water, adding gypsum is likely. The use of a portion of the alkaline tap water should enable you to dial in the appropriate level that meshes with the high hardness you create with the gypsum. Using 100% RO along with the high hardness is likely to drive the mash pH too low, so the use of a small portion of tap water could be useful. That is where Bru'n Water is helpful for calculating the percentage.
Using Campden tablets will be needed to remove the chloramine from that tap water component. The rate is 1 tablet per 20 gallons of tap water. You don't need to add any for the RO water.
I recommend adding gypsum to produce at least 100 ppm sulfate in the brewing water. That level can go as high as 300 to 350 ppm, but is based on your tastes. I like 300 ppm, and found 100 ppm just barely adequate.