It's determined in the same way that "safe following distance" is determined. If there's an accident that you could have avoided by being farther back, then clearly you were following too close. If you get hit right after rolling through a stop sign, then you didn't slow down enough.

I think that's what noglider meant. As long as you feel safe, go for it. If you don't get in anyone's way and no accidents happen, then you're right. If something does happen, then you're wrong. If you want to protect yourself from being second-guessed, then stop.