Size-based spot modifiers should really be multiplicative.

Something twice as big should be visible twice as far (barring issues like atmospheric interference).


Spotting a light is goofy too. Visible at 20x the light radius. Which isn't even close to accurate. In good conditions (clear dark night) a candle is easily visible - as in it will catch your attention when you aren't looking for it - at a mile. Optimum for human vision is something like 25 miles. So something between 100x and 10,000x the light radius.