The past two days I've brewed a kolsch and a marzen, targeting a mash pH of 5.6-5.7, as measured at room temp, which in principle puts me in the ballpark of 5.40 if measured at mash temp. I used tap water, which in my city is very soft: 7ppm Ca, 2ppm Mg, 5ppm Na, 2ppm SO4, 5ppm Cl, 28ppm HCO3.
The kolsch was 97% Weyermann pilsner and 3% Weyermann vienna. Estimated SRM of 2.7. The mash pH hit 5.67, without any salts added to the strike water. This is an extremely pale grist, and I was able to hit the optimal mash pH without any added calcium.
The marzen had a more kilned grain bill, and the no-salts mash came in at 5.45 (again, measured at room temp). I added a little baking soda to nudge it upward to 5.62.
I apparently will never have to add calcium to hit my target mash pH, even for the lightest grain bills in terms of SRM. I find this conclusion odd--hitting the ideal mash pH for what is practically an all-pils grain bill without any salts intervention--but I can't argue with the numbers.
I definitely needed to, and did, acidify the sparge water some, and will need to more, as I came close ph 6 and gravity 1.010 on the last runnings.
Oh, and that line equation from Narziss was spot on for both brews.
I ordered the cheap, expendable pH meter recommended by dmtaylor. I will use it to measure mash pH at mash temp so that I can get a sense of the offset between that and room-temp pH.
Lastly, for the experts: by not adding any calcium salts to the mash, am I risking beerstone more than I otherwise would? I do add salts to the kettle, but obviously I'm bypassing any oxalate precipitation that would occur in the MT with added calcium.