As we know radius ratio for cubic void
i.e., r+/r-=0.732 (without distortion)- 0.999(with distortion)
if R is r-, then maximum radius of r+(which is the radius of cation occupying void) is 0.732 times of R
for metallic solids:
2R + (empty space) = √3a
empty space = √3a - 2R