I've found the EZ Water Calculator spreadsheet works a treat. It highlights the need for further acidification, if needed, after you've entered your salt additions.
There's a box for 88% lactic acid. I use 85% phosphoric instead but the same amount. The mash pH (measured at room temperature) is 5.2 just as predicted. In the California Common I made yesterday, I needed to add 3ml of phosphoric acid, despite reasonable salt additions. So you can see that we're not talking about tiny tiny amounts. I suppose, thinking about it, adding acidulated malt is even more stress free since your LHBS can do that, no need to try precisely measure out phosphoric/lactic acid. EZ Water Calculator has a box for acidulated malt, bosh.
Not trying to place myself in opposition to Thirsty Boy, but I'd like to comment on the half a teaspoon sort of thing because it's how I started off with water additions. First off, I weighed a teaspoon of each, because they weigh differently. A teaspoon of CaSO4 is about 2.8g, a teaspoon of CaCl is about 3.4g. Using my Common beer yesterday (1.052OG, 25EBC) as an example, working on the actual water quality report* from Melbourne's Cardinia reservoir, entering the figures into EZ Water Calculator... Half a teaspoon of each for a balanced beer. The mash pH would be 5.57. Three salts out of spec according to Palmer's guidelines:
Ca - 27ppm (Palmer 50-150)
Mg - 1ppm (Palmer 10 - 30)
SO4 - 24ppm (Palmer 50 - 350)
Clearly it makes good beer or Thirsty Boy and countless others would have noticed by now, and my prior beers weren't half bad either. Yet it does seem to be considerably out of range, particularly for mash pH. To be in range, I had to add 4g MgSO4, 6g CaCl and 3g CaSO4. That's about 1 tsp, 2 tsp and 1 tsp respectively. And the 3ml of acid in the mash to hit 5.25 (and measurement confirmed) as mentioned. Obviously different beers would be different, this is just an illustration.
felten linked a forum post (cheers!) where deLange cleared up the issue regarding pH and measurement temperature. It seems that you do actually want to measure it at room temperature. So when people talk about an ideal pH of 5.2, they mean room, not mash temperature. Phew. He goes further and gives his measured pH/C change as 0.0055pH per degree. 66-21 = 45. 45 * 0.0055 = 0.2475. So if the mash is 5.2 at room, it should be fractionally under 5 at mash temperature.
If you read 5.2 at mash temperature then that's 5.45 at room. Which is no disaster anyway, it seems.