Adjusting water for RIS

Im planning on boiling a 10%+ RIS for long term storage. So long story short, Is 10g baking soda and 10g chalk in a 23L mash (2.3L per Kg) and 8g baking soda and 8g chalk added to the boil, total pre-boil volume of 30.44L and final volume of 23L (if all goes to plan) Estimated colour is 72.9SRM. According to Palmer's recommended ranges my calcium is too high at 184ppm (150 maximum recommended) but I think I need this to reach an acceptable pH, this will get me up to 5.21pH according to EZ water calculator 2. Will the additions be too much? Any other suggestions on how to adjust my water or is it just not going to be possible for my to brew something so dark that will work out properly? Any more info needed let me know, just trying to get the main info in the first post.

Cheers

Is a total of 18g each of chalk and baking soda going to make the brew taste, well "chalky"? Any other suggestions for how to adjust the water?