Well, when you're first adding it, remove the water for your water change as usual. Take a bucket and add all of the mix to treat 25% of the tank to that one bucket. Then add water, and dechlorinator, and add it to the tank. Then refill the tank as usual. The next time you do a water change, do the same thing, and so on, until you've got something close to 100% of the water "treated" with the mix. You'll be able to monitor KH and pH during these times so that you can see what's happening.
If you get to where you want to be before you get to the 4th water change, just don't keep adding the mix. Have you only treated half of the water with the recipe suggested? Then base your further additions off of that.
So take whatever you end up with and are happy with, and then use it to determine how much you're adding during regular water changes (when all of the water is treated to your liking). If you do 25% water changes, and your tank is a 125, then you're going to come up with the numbers to treat 30 or so gallons. So, if you're doing the 1/1/1 per 10 gallons, because that's what you decided worked, then you'll be doing 1.5 teaspoons of salt mix and baking soda, and 1.5 tablespoons of Epsom salt for that particular water change (replenishing what you're removing). Then you add that the same way -- add it to only one bucket of the new water, stir, add dechlorinator, and put it in the tank, completing the rest of the water change as normal. You'll do this every time you do a water change in order to keep those KH and pH levels stable.
In case this only makes sense in my own head, basically what I'm saying is that you add gradually until you get to where you want to be, but keep up with what you're doing so that you know how much to dose to keep the levels steady. And you only have to use one bucket of water add to the tank -- just enough to dissolve the mixture before you refill the tank. This is how we do it and it works really well, so I hope it works for you!