I usually just use the 2-second rule, that's how I determine if I'm too close or not. At any given time, say doing 60 down a highway, I select a given point (say, a no-passing zone sign) and count two seconds out in my head. If I reach that same point before the 2 seconds is up, I know I'm too close. I'm pretty sure that's also what's recommended by the DoT, at least in the state of Iowa.