If the pool store tells me to add 20 pounds of Alk Up, how much Baking soda do I add?
Answer....Zero

Before I did what the pool store said, I would confirm where my alkalinity is and where I want it to go.

Figure the gallons in the pool, Test for alkalinity, and then add enough to get you to 100. If you've already done that and the pool store is correct, the answer is 20.

I would base my dose on how much I'm trying to change it. A 40 point increase in a 24k pool is 13.5 lbs. At one pound per day, you would take two weeks (and 14 applications) to get there.