There is a very nice primer in the brewing science section for water additions.
It's pretty simple, and the hardness isnt everything that matters, it's more the alkalinity and what makes up the hardness that is the real issue (they have a larger impact on mash ph).
Here, it's just best to read the primer: http://www.homebrewtalk.com/f128/brewing-water-chemistry-primer-198460/
As AJ would say, do the above, but test with a ph meter during your mash (wait at least 15 minutes into the mash to let the PH settle and cool the sample to room temp) and have it guide you more than anything that is said. I've found in my own thornton water around here that 2% acid malt gets me too low, but 1% makes me hit the target just right, however 3% for a pilsner got us very close going to move to 2.5% or so to hit the target (there is a lot of different numbers thrown out, I usually shoot for 5.4 - 5.5 ph at room temp).