As we know,
for such structure
2r⁺ +2r ^– = a
Also, as we know,
density = nM / VN₀
here, n= 4
2.48 = 4 * 58 / (a)³ (N₀)
distance between K⁺ in KF = a/2 = 269 pm

For NaCl : Distance between two nearest ions (r⁺ + r ^–) :

2r+ + 2r ^– = a

i.e.     ${\mathrm{r}}^{+}+{\mathrm{r}}^{-}=\frac{\mathrm{a}}{2}$