How to calculate Debye length of a 2-1 electrolyte
If we follow the derivation of the Debye length model, we would arrive at the following adapted model (doesn't assume uniform charge distribution):
$$\lambda_{D}^{2}=\frac{\kappa k_{B}T}{4\pi Z_{1}Z_{2}e^{2}c_{0}k_{c}}$$

In our equation: 


$\lambda_{D}$ is the Debye length;
$Z_{1}$ represents the charge on the cation; 
$Z_{2}$ represents the charge on the anion;
$\kappa$ is the dielectric constant;
$k_{B}$ is the Boltzmann constant;
$T$ is the absolute temperature; and 
$c_{0}$ represents the number of ions per a cubic metre of solution. 


Note that the general equation assumes that $Z_{1}=Z_{2}$, hence why this is usually written as $Z^{2}$. 

Keep in mind this is only applicable for calculating the Debye length for a single dissociated species in solution! Further alterations would need to be made in order to consider a mixture.