I'm finding that residual alkalinity is only a component to understanding and predicting a desirable mash pH. The beer color versus residual alkalinity recomendations that I've made in the past and many water calculators use, are woefully inadequate.
It turns out that the variation in grain acidity that Kai deciphered over a year ago is very key to the refinement in our ability to assess what might happen in the mash before we actually brew. To brewers that don't have pH meters and the time or inclination to fine tune their brewing water, there is the potential that better water calculators can be devised.
Another thankful property of our brewing grists appears to be its tendency to buffer the over application of either gypsum or calcium chloride and not push mash pH too low. I've complained in the past about brewers that espouse creating water with negative residual alkalinity. My research suggests that the mash buffers prevent these mineral additions from pushing pH down. So, that doesn't matter too much. If you want to create negative RA brewing water, it is OK, but it doesn't really do anything extra for the mash.
Mash water chemisty is still very complicated, but hopefully we will move beyond the misinformation that is out there now and devise tools that a regular brewer can apply. I'm trying and I know there are others.