I've never built my water, but this is how I would do it. Read Chapter 15 of How to Brew. After you understand what each salt does, then I would download the spreadsheet on the bottom of this page. Then play around with the spreadsheet, leaving everything in the source water profile as zero since it is RO water. Step 5 is the one you want to look at the most. Play around with the salt additions until you get the profile you want. Palmer lists some common water profiles of major brewing cities in Chapter 15 of his book. Hope this helps.I've used RO with great success on extract batches with adjuncts. However, I too am very concerned with my jump to all-grain. I've asked on the forum before and nobody seems to know exactly how to compensate for the lack of minerals. How much gypsum do i add to 6.5 gal? how much 5.2 do i add? (if any) these are the questions that i would want to know. If you could pin-point how much of these minerals and additives to add to the water to create certain profiles then it would be foolproof to make perfect water. However, I think that this might cost more because you would be buying the water in addition to probably adding more salts/nutrients to the water as opposed to most brewers who already have some amount of this in their water. Discuss.