If we apply this formula to scattering of neutrons by the atomic nuclei, we see that the range of is much smaller than the distance between atoms. This is a simple number which we can look up in tables. If we make the wavelength of the neutron much smaller, and the energy larger, we will see details in the scattering density due to the structure of the nucleus. But than we are doing nuclear physics, and not solid state physics. It is all we need if we want to study crystals, unless we need the absolute intensity.
Absolute numbers are hard to get, one has to correct for all kinds of experimental details.
Hence if we investigate the diffraction peaks we see zeros where we expect peaks. This is an indication that our symmetry is larger than what we assumed. We knew this, of course, since our crystal is really fcc which has only one atom in the unit cell. Note that the RL of fcc is bcc, and that a bcc structure in terms of a simple cube consists of all vectors with all indices either even or odd. Then the atomic form factors are different and they have to be included.