A couple tips from Martin about when to add stuff....
It does make a difference when you add any acid to brewing waters. The alkalinity value that was reported for the tap water was measured in the water near room temperature. As pointed out above, raising the temperature of water reduces the solubility of dissolved carbon dioxide in the water. This decarbonates the water and converts alkalinity in the water (mainly bicarbonate) to undissolved chalk in the water and the carbon dioxide bubbles out of the water. The alkalinity of heated water is reduced in comparison to its room-temperature alkalinity.
Since acid additions are calculated based on that reported room-temperature alkalinity value, it is important that any acid dose calculated for that water be added prior to heating the water. If the acid is added after the water is heated, the reduced alkalinity of the heated water means that too much acid will be added and the resulting brewing water alkalinity will be lower than predicted. Add all acid to brewing water prior to heating in order to achieve the alkalinity targeted by brewing water calculations. If acid must be added after the water has been heated, test the alkalinity of the heated water and use that new alkalinity to calculate the acid addition.
Gypsum is more soluble in cool to warm water (maximum gypsum solubility occurs at around 40C or 100F) and should be added to water prior to boiling for quicker dissolution. Vigorous stirring is typically required to speed the gypsum dissolution. If gypsum does not dissolve in a timely manner when added to water at less than 1.6 grams per liter, test the gypsum for the presence of chalk by adding an acid such as vinegar or lactic to the dry powder. If the mixture 'fizzes', the gypsum is adulterated with chalk.