My original experience with a substrate that removes carbonates was in 29 gallon tanks with a Fluval 204 canister, and not much substrate or decor. I was calling the volume 30 gallons. I also had SMS in several other tanks from 10 gallons to 45 gallons. Varying amounts of non-water stuff taking up the room in the tank.
At the same time I was running a Lake Tanganyikan tank of about 60 gallons (Sump + tank, but there was a lot of rock in this one. Actual volume was closer to 50 gallons). I needed to prepare the water for water changes by raising the GH and KH of the tap water. I would mix a garbage can full and use it for several tanks with hard water fish.
Soil Master Select as the substrate in the 29 gallon tanks would remove the carbonates, per API liquid test and dip stick test (Probably Jungle, but there are several companies that make them). As near as I could tell the KH was 0 degrees (0 ppm on the dip stick test)
When I added 2 teaspoons of Arm and Hammer Baking Soda to the 29 gallon tanks these tests would both show 2 dKH. I realize the dip stick does not have a "35 ppm" color, so this is interpolating between the colors. It was really close to the 40 ppm shade of green.
I run SMS in many tanks of different sizes, and multiply or divide the dose to get the results I want depending on the size of the tank.
I got the same results when I was preparing the water for the Lake Tang. tank. 30 gallons of new water, starting with tap water KH 4 degrees. Add 2 tablespoon and the KH would rise to 16 degrees.
Then I set up an 88 gallon tank with Safe T Sorb. I found it also dropped the KH to what I called 0 degrees.
I added varying amounts of baking soda and tested the results. I found the math holds true:
1 teaspoon of baking soda added to 30 gallons of water raises the KH by 2 German degrees of hardness.
I also altered the test by mixing 50/50 distilled and tank water then multiplying the results by 2. I was measuring the water to fill the test tube using a syringe with markings to .5ml.
You can multiply or divide this recipe to suit any size tank, any desired change in KH.
However, if I was going to attempt a much larger change I would probably work it part way, then test again. I was not measuring the baking soda by weight, which is the more accurate way to do this sort of thing. I know when I got a bit careless mixing water for the hard water tanks the KH did not exactly match the tank.