What does it mean to calculate hardness in terms of ppm of calcium carbonate?
It is meant as molar equivalent. $\ce{CaCO3}$ has the molar mass about  $\pu{100 g / mol}$, $\ce{Ca(HCO3)2}$ has the molar mass cca $\pu{ 162 g / mol}$. Therefore $\pu{100 g}$ of the former is equivalent to $\pu{ 162 g }$ of the latter. The similar for the concentration expressed in ppm. E.g. if water contains $\pu{1.62 ppm}$ of $\ce{Ca(HCO3)2}$, it  is equivalent to $\pu{1 ppm}$ of $\ce{CaCO3}$