Its easy to judge a tail light. Since a car approaching from behind you could be coming at 40 MPH or so you should figure how long it would take a driver to recognize there is an obstacle up ahead to avoid, add that to the amount of time it takes to actually maneuver the automobile away from said object and then figure out how far a car going 40 MPH would travel in that time span. You'd be surprised at the distance all this takes (about the length of three telephone poles). And that is how far away you should be able to see and identify a tail light as a tail light, if you want it to help you.

The second surprising thing is how many bicyclists judge the brightness of a tail light by standing right behind it (or while holding in their hand), they are all bright at that distance.