If both your K2006 test kit and the guess strips match, ignore the pool store.
I bet it was some kid using strips. Sorry I suggested it. I have never heard of anyone getting a 0 FC reading and a positive CC reading. if TC is 1.8 and FC is 0, then C must be 1.8. I don't believe their numbers.

BTW, the "Alkalinity Plus" is very, very, very, expensive baking soda. Sodium bicarb, but they use an alternate name, Sodium Hydrogen Carbonate, which is the same thing, just an elaborate attempt to fool you.

Before you waste money on calcium and risk going 'way over 400, invest in new reagents first.