I only use RO water and from what I have read on HBT(As a general rule of thumb), and mainly from Adjelange, is to start with a tsp of Gypsum(~4g) and a tsp of Calcium Chloride per 5g of RO water. Then add about 2-4% of Acidulated Malt to bring your PH in the desired range(5.4-5.6). And depending on style you can adjust things from there.
I use RO for many reasons, but it is easier IMO to keep things consistent. Aside from removing the Fluoride and all the other crap that is in our city water. Just don't want it in my beer.
Anyway, I am pretty new to water chemistry so hopefully Adjelange or one of the more experience ppl will chime in and make corrections. It is more work obviously, but your water is soooooo important to brewing a good beer and being consistent. That, and a good PH Meter and wort at room temp for 15-20 min.
Primary: Reaper's Mild
Kegged (Drinking): DFH 120 Min IPA Clone, Hefe
Bottled: Cinnamon Vanilla Mead, 8-8-08 RIS, Devil May Cry, Westvleteren 12 clone, Very Berry Mead
On Deck: How Rye am I Saison, DFH 120 min clone