How to know which ionic compound have the highest lattice energy without knowing the values of the radii? Given two ionic compounds, for instance:

$$\ce{CaS} \quad \mathrm{or} \quad \ce{KCl}$$

What is the procedure to predict which of the two have the highest lattice energy (in absolute value)?

(Suppose that I do not know the values of radii of the atoms but I can only look at the periodic table).



I know that I must look at the ratio 

$$\frac{Z_1 Z_2}{r_1 + r_2} \propto E_\text{lattice},$$

where $r_1 + r_2$ is the distance between the two atoms in each molecule of the compound, $Z_1 Z_2$ is the product of atomic numbers of each element in the molecule.

But, for example, in the case above I have both $r_1 + r_2$ and $Z_1 Z_2$ bigger for $\ce{KCl}$ then $\ce{CaS}$, so I cannot say (without knowing the numerical values) if one of the two ratios would be higher.

Am I missing something?
The Lattice Energy of Ionic Compounds is directly proportion to the Charge Density. As Ca, S have 2 units of charge while K, Cl have one and there is not much of a difference in size ( atleast not enough to overcome the doubling of the charge), the charge density of Ca and S ions would be more. Consequently, the lattice strength of CaS would be more.