Best way to reduce alkalinity is to add the product to a still tub: Don't run the jets. You'll get more out of it if you do. Take your water to a professional and have them look at the level, test it and add the recommended amount. You may have to add, say, 1/2 to a pound to of pH lower to reduce your pH.
Once you get it down, keep a mental not of what quantity you needed to add to get down to 80-120ppm and then when you refill, add that amount to the footwell while the tub fills. It makes life so much easier.