Here's the bottom line: What do YOU think is a safe distance when you are at a red light? Consider these things before you answer:
1) at whatever distance you choose, do you have enough distance to move out of the way if you see someone behind you isn't going to stop in time?
2) if someone rear ends you from a reasonable speed while you are stopped (reasonable meaning they were trying to slow down and stop), do you have enough distance between you and the car in front of you to not hit them?
If you feel your distance satisfies those two things, then that's a safe distance. Safety while stopped relies on too many things to "define" it as any distance. Certain distances can be rules out, obviously, such as 2 inches from the person in front of you, but other than the obvious, it's up to your judgement and what you deem safe.
1) at whatever distance you choose, do you have enough distance to move out of the way if you see someone behind you isn't going to stop in time?
2) if someone rear ends you from a reasonable speed while you are stopped (reasonable meaning they were trying to slow down and stop), do you have enough distance between you and the car in front of you to not hit them?
If you feel your distance satisfies those two things, then that's a safe distance. Safety while stopped relies on too many things to "define" it as any distance. Certain distances can be rules out, obviously, such as 2 inches from the person in front of you, but other than the obvious, it's up to your judgement and what you deem safe.