Yes on the water report input tab you can leave that blank and then on the water adjustment tab, select RO under dilution water and set to 100%. The instructions page of Bru'n Water states this:
"If Reverse Osmosis (RO) or Distilled Water is the brewer's primary water source, then input on Sheet 1 can be skipped and the brewer can move directly to Sheets 2, 3, and 4. Select either RO or Distilled from the Dilution Water Profile selection and set the dilution percentage to 100 percent on both the Sparge Acidification and Water Adjustment sheets. The water adjustments can then be properly entered to create the desired water profile."