Energy is only "lost" as it is absorbed by the matter it is passing through (air, usually). In a vacuum no energy would be lost, but the beam would still spread out.
Depends on the focus of it. You can determine the power by doing:
Order of magnitude of the beam's solid angle is approximately wavelength/area of the laser's output aperture is approximately (the vertex angle of divergence)^2
This will tell you it's spread and therefore you can calculate the beam's focal point and therefore at what distances you are receiving full power in a vacuum
An example is:
wl: 500nm
Area of aperture: 5mm^2
So:
Order of magnitude of the beams solid angle (estimate sign) (500)^2*10^-18 m^2/5^2*10^-6)=10^-8 so the vertex angle of divergence is 1/10 milliradians
That is just for checking the power without the lens or an lpm
Hope this helps
PLEASE correct me guys if this is the wrong formula.