Light Falloff Calculation

What is the base of the Light Falloff Calculation. Is it based on Blender Units?

If yes it I should be able to calculate the Light Value at a specific distance like in the attached image?

The distance part in falloff is not really tied to any specific units. The inverse square law describes how the relative irradiance changes: twice as far, surface of same area receives quarter the light. It is due to simple geometric principles. But using special shapes of emissive objects or scrims, aparent light falloff can be changed: think of egg crate diffusers for example, that apparently break the falloff by varying the occlysion of light the farther from light the object is.

If you want to calculate real irradiance or flux or some other radiometric unit, one blender unit can be assumed to be 1 meter and light intensity values for emissive surfaces are measured by watts per sq m of emissive surface (radiosity). For point lights it is watts per total emission (radiant intensity multiplied by light emission area in solid angles), for sun light watts per sq m on incident surface (irradiance). You should be able to calculate the irradiance values based on this.

If you want more precise control over the light falloff, then you can take a look at Cycles’ Light Falloff node (be warned though that it can cause incorrect light decay if not used carefully).