Lattice energies for different compounds and Coulomb's law?

Google Born-Lande equation
The largest term in the theoretical calculation of lattice energies is the Coulomb attraction term that depends on (Z+× Z-)/(ro) where Z+ and Z- are the charges on the cation and anion, respectively (the larger the charges the greater the attraction, the greater the lattice energy).  ro is the distance between the cation and anion considered as point charges.  The shorter the distance the greater the attraction.
ro = r+  +  r- where r+ and r- are the radii of the cation and anion, respectively.  The Z+× Z- term dominants; if the Z+× Z- values are equal then consider r+ and r-.  Let's first sort the cmpds according to Z+× Z-:
NaF Z+× Z- = [1]× [1] = 1
KF Z+× Z- = 1
KI Z+× Z- = 1
MgCl2 Z+× Z- = 2×1 = 2 second highest U
MgO Z+× Z- = 2×2 = 4    highest U
Now for the M^+X^- ions:
rNa^+ <  rK^+      so U(NaF) > KF
rI^- > rF^-    so U(KF) > U(KI) you can google and check their mps (proportional to U) I hope MgCl2 is not an exception.
So order of increasing lattice energy is  KI < KF < NaF < MgCl2 <  MgO
cheers.