I think it is prudent to hold some out and add it in if and only if necessary. The problem with doing this is that the pH reading takes about 15 minutes to stabilize when sauermalz is used so don't take the first reading as an indicator of where you are. If you read, say, 5.2 initially wait to see how much it increases in 15 - 20 minutes before deciding whether to add the rest. You should only have to do that once or twice. After that you will know how much sauermalz is required and add that much at the outset (but still check with the meter).