I just served as lead judge for Amber Hybrids at the NHC Midwest regional and sampled some fine D Alts. Given the need for assertive bitterness, I think you would be better served with a stronger sulfate bias. I do agree that keeping the overall ion content somewhat low is a good idea. I'm not sure its necessary to bump the Ca content as high as indicated, but it probably won't hurt.
Another option for assessing your water adjustments would be Bru'n Water. It includes a large set of balanced and verified water profiles to help you decide on your water adjustments.