LED visibility calculation
LED brightness is normally in lumens at one meter/one yard, so that is your baseline for intensity vs. distance.
Normally for radiant light, the intensity drops to 1/4 of original value every time you double the distance. For a given intensity at 50 feet, it will be 1/4 that value at 100 feet. It will be 1/16 that value at 200 feet. To see an LED 500 feet away you may need 50,000 lumens. Our eyes see green and yellow light the best. We do not see blue, red or violet very well.
I hope this helps in some way.