Wow, this is great. I'll be using this for my next brew, a traditional Irish Dry Stout (I know, a bit late for the season).
Water Profile from Ward Labs:
Since it is 70% Marris Otter, 20% Flaked Barley, and 10% Roasted Barley, then the roasted percentage should be 100%, correct?
According to the calculator, then I should hit a mash pH of 5.5, which is perfect.
I'll give this a go when I brew it sometime this week. I only have ColorpHast strips, so the mash pH test won't be that accurate (even accounting for the strip error), but it should tell me if I'm in the ballpark.