Hey ,
The lighting is based on the vertices of your mesh and not the pixels when using a transparent material. In turn, if you are using a large mesh, like your plane, that is tessellated to have accurate lighting. The light emitting in the distance will still be attempting to accurately light the vertices in the distance.
Your information is correct in regards to per pixel shading, but this equation does not apply and scale the same as vertex lighting. I did find a solution which you might be able to apply to this specific set up.
Light Functions
With a light function you can set the Light function fade distance property, which seems to be what you are looking for in regards to your own project.
Cheers,