Knowing the pH/alkalinity of the source water is key. Additionally, if you use, say, a total of 16oz (1lb) of reducer to finally get your pH and/or alkalinity in range, note that for the next time you fill the tub. You can add the total amount to the footwell when you begin to fill the tub and by the time it finishes its first jet cycle the water should be balanced.
Additionally- I have found that when lowering pH and/or alkalinity, it is most effective when the water is completely calm. Running the jets is fine, but the most effective way to lower is to have the water completely calm.
I have some customers that require 1-3lbs of reducer from the onset as their source water is high in both pH and alkalinity. So don't be afraid if you have to add a bunch of reducer when you fill your tub.
I hope this helps. Good luck moving forward.