pH correcting the mash of very dark / high SRM beers

I understand that darker malts will lead to a more acidic mash, and that to raise the pH you need increase the residual alkalinity by adding sodium bicarbonate or calcium carbonate. But for extremely dark beers, like an imperial stout, you have a lot of residual alkalinity to add. If I have a 40 SRM beer (in this case, an imperial porter) the residual alkalinity calculator shows I need to target ~400 RA in my beer. However, to get that much RA in there, I need to add so much sodium bicarbonate or calcium carbonate it would raise the sodium or calcium levels way beyond their recommended maximums.

So how do you guys handle this? I'd imagine you'd add baking soda first to the maximum sodium range, calcium carbonate to the maximum calcium level, and past that? Do you leave the mash slightly lower? Push the sodium or calcium even higher? Use some other solution to get you to 5.2?