You'll probably have to do a ratio of liquid chalk (your tap water) to distilled water. I've had troubles finding the information I need to properly fill out the Bru'n water spreadsheet, and those figures you get from your water company aren't that accurate. By law, they only need to test and report on their water quality once a year. What they don't take into account are rainfall amounts, seasonal dilution and concentration, spring and summer farm runoff, etc.. There's a whole slew of factors that go into your daily water quality.
I found the best way for me to deal with it was to do a 50%/50% mix of distilled to tap water. I took what information I could find about the local water and divided it in half, which was perfectly in parameters for what's considered ideal brewing water (see the suggestions there or in Palmer's works). Eventually, I invested in a Kent Marine Deion 200R filter for our aquariums and it produces virtually 99.9% pure water. For us, it was a better option than reverse osmosis.