Only takes 1 oz of acid malt to get your pH in range?!
The water in Germany, according to most of the info in Brunwater, is aparently pretty soft. So I don't think you need add much to be honest. Maybe boost your calcium if you want it to be crystal clear?
For me water additions are mostly about pH balance. I add sulfates to bitter beers and since my water is on the harder side seldom need to add calcium for clarity.
Looks like an interesting recipe. Is the chocolate malt for flavor or color?