Many RO users like to blend in 10% or so of their tap water just to be sure they have trace minerals for yeast health. This really isn't necessary because malt contains as many trace minerals as yeast really need (possible exception: zinc) but I do it. Your problem is the high alkalinity of your water - approximately 250 ppm as CaCO3. Even with a 9+1 dilution your water would have alkalinity of approximately 25 ppm. You don't want it much more than that.