Equilibrium constants for lewis acids such as Ca2+
You're confusing terms here. $\ce{K_{a}}$ is the (Brønsted) acid dissociation constant. This suggests that $\ce{K_a}$ refers to the degree that a Brønsted acid dissociates into its conjugate base and hydrogen protons. 

$\ce{K_{a}}$ is inapplicable to Lewis acids because Lewis acids are not proton donors. 

Brønsted acids are Lewis acids in that Brønsted acids such as the hydrogen proton can accept a pair of electrons. But the converse is not true. 

Lewis acids are electron acceptors. How do we measure the strength of Lewis acids? There is no universally accepted measure as there is for Brønsted acids. We could look at the enthalpy of reaction (Lewis adduct formation) between a Lewis acid and a Lewis base. This is a common method.