One can estimate energy intensity for line of sight radio communication using the inverse square law.
Assuming perfect reflection from a flat surface, is there any formula, or is there a way to estimate energy intensity for radio waves that reflect once from a surface before reaching a destination?
A "flat perfectly reflective surface" is known as a mirror, and all that does is change the direction of wavefront propagation. It has no effect whatsoever on the wavefront profile (for you nit-pickers, I'm assuming a bulk reflector, not a multilayer dielectric with polarization sensitivity).
Now, if your actual question involves the interaction between a beam which has both LOS to the target and a reflected section, then you have to deal with interference effects. That's a more complicated situation.
