shock n swim to chlorinate pool if my calcium is low?

You have it figured out!

BBB is about using what's appropriate, in the appropriate amounts, knowing the possible side effects.

Use it. Pool Calculator can crunch the numbers for you under "Effects of adding chemicals" so you'll know how much you can safely use. If it's okay to use 12 pounds and keep CH in range, no point buying 20 pounds. But if I could get away with it, I sure would. Easier to store, easier to carry, long shelf life.
 
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