The amount of additive you use depends on your grain bill. You want a mash ph of between 5.2 and 5.6. Darker grains are more acidic. I use something called EZ water calc. It's an excel file that let's you enter your grain bill and adjust water ratio and additive amounts until you hit your target...
Palmers "how to brew" has a section on water chemistry and mash ph that finally unlocked it for me. The online version has a excel spreadsheet and a simple sheet that really makes sense. The 5.2 additive has a lot of salt, so depending on your water chemistry, it may not be a good idea. Google...