After reading this terrifically long thread I have a couple points.
My first point is that pH is not the only part of water chemistry that effects beer flavor. This is really important, because it seems like most of the time people are thinking just about mash pH. It was mentioned already but I want to mention it again. The grain, and intended color of the resulting beer, is tied to the mash pH. But flavor is affected by several minerals, most importantly Sulphate, Chloride, and Sodium.
As for target profiles while some beer styles have prototypical target profiles, I personally just use the "Brewing Range" description of each mineral in How to Brew to come up with target ranges for each mineral based on the flavor I want out of the beer I am brewing. You don't even have to understand anything about the science behind it to just copy down the range based on the flavor you want. The point is to think about the resulting flavor you want.
Now, having said that, I find the
Brewing Water Chemistry Calculator | Brewer's Friend most helpful because it not only addresses pH and residual alkalinity but also target flavor, target color, and it also provides visual indications of when you are in range with your target profile as well as providing visual indication when you have added too much of any particular mineral. It also lets you specify a dilution percentage.
My last point is regarding the additions schedule. I think the best advice I have come across is to do your additions in two steps. First, come up with the correct amount of additions for you mash volume. Then come up with the correct amount of additions for your sparge volume. Add the mash additions to the MLT (not the strike water kettle) and add your sparge additions to the wort in the boil kettle (it is not necessary to add it to the sparge water). If you add the additions to the strike water kettle instead of the MLT, the temperatures are high enough to cause the additions to precipitate - defeating the whole point of adding them in - I found that out the hard way. It is true that the pH during the sparge is not that important, but remember, additions are not just about pH but also about flavor. So you want to make sure you add the correct amount to get the mash pH correct, but also make sure you add enough full boil volume to address flavor. This is the method I have found the most success with.