Let me attempt answering this one... I guess Martin will correct me if I got it wrong.
In the calculator, the alkalinity used for the sparge acidification should match the alkalinity of the water you plan to sparge with.
You may choose to use un-treated water, in which case you'd use the data in the water report or you may choose to use treated water, in which case you'd choose the value corresponding to the finished water. Or better yet, the result of a KH test done on the brew day!
Now, salts that affect (add) alkalinity (such as baking soda) are not recommended in the sparge anyway, so ... If your water treatment includes either of these salts, you are probably better off sparging with un-treated water. Or adding these salts only to the mash.
If your water treatment does not include alkalinity changing salts, then is up to you to choose if you want to use un-treated or treated water. The advice is that, if you choose to acidify with treated water, add the salts prior adding the acid and then heat up the water. I personally prefer not to add salts to the sparge water, and instead add what is missing directly to the boil.
Alternatively if you have a pH meter, you can add the acid a fraction at a time and monitor until you hit your pH.
As per your second question, if you have a target pre-boil pH for different recipes, then the acidity of the sparge water may help you hit that pH. I think though this is not something people will attempt calculating as you'd have to understand the buffering capacity the grains, wort from previous runnings, etc.
If you are interested in tracking this, however, the best advice would be to keep good records. Measure your pre-boil pH and if you didn't hit the target by far (for me that is +/- 0.2), you can make adjustments (if you dare) or wait until next time you brew the beer, taking in consideration that the ingredients next time round will not be exactly the same.
BTW, I've only brewed three times. So we may both wait for someone else to aswer.
