Distance X is going to be determined by the lumens that the light source emits. After that it's a mathematical formula which might be a bit over the top for your purposes. But if you have a 3d package you could set up a simple test scene with physical lighting and real world dimensions to study the falloff based on lumens…
Haha! Sorry I mixed up twice in one post :) I thought you said you are a Vray specialist. And yes, of course you're correct, I meant the other way around. Larger light higher multiplier to account for the lumens having a larger, less concentrated area to cover. I don't paint in 2d but I can imagine how difficult it would…
Well if you're a Vray specialist then you'll already know that every time you double the surface of an area light you increase the multiplier x4 to maintain the same luminence. This is essentially the inverse square law for artists. Test it yourself. Just set up a plane for the ground and 2 plane area lights: one with a…