The PH of the RO will not change until it is exposed to air. It is the absorption of Co2 that lowers the PH. A gallon at a time is a PITA. I fill (make it) a 5 gallon bucket with RO and ad the Cal-Mag to it then add to the reservoir.
Did you ever watch a chef at a Chinese restaurant add sauce to a wok? No they add sauce ingredients, it is part of the wok cookery.
Add oil to wok get hot if using marinated meat cook it partially first, then add mushrooms for a minute then add all of your vegetables Soy sauce and fish sauce, sugar and any other spices you like and stir fry until nearly done. The sides of the wok are where the most of the cooking is going on. There will be plenty of juices on the bottom of the wok by now. Then when nearly done add some corn starch (dissolved in a little cold water) this forms the sauce just keep stirring.
edit: I think by sauce in a cup you may mean the raw sauce ingredients so they can be added to the wok in one easy pour. Most of the time I only use Tamari and a dash of fish sauce and I can just add from the bottle.