To add to Martin's comments, I add the mash salts to the mash. I also calculate the difference between the mash liquid volume and the kettle full volume and add the "sparge" salts to the kettle when full.
For example, if I have 8 gallons of mash liquor and the kettle full volume is 13 gallons, I calculate the additions for 5 gallons of water and add those to the kettle. I leave out the salts like Baking Soda (Sodium Bicarbonate) and pickling lime (Calcium Hydroxide) from the kettle addition just like Bru'n Water specifies for the sparge water. I don't use chalk (Calcium Carbonate) anymore because it is too hard to dissolve. Hope this helps a bit more!