^^^ Unfortunately, this is not always true. One of the most common nasty things that arises when doing coke/crack are seizures. You can be feeling fine, then take a hit and start convulsing out of the blue. I've seen it happen.
I don't know of any way to tell where the 'too much' level is for a given person without hitting it. I would watch out for signs like uncontrollable trembling, rocking back and forth, and the visual hallucinations (eg seeing things creep towards you) -- these usually happen just short of the danger area.
Also, be VERY CAREFUL during binges. Even once the crack has worn off, it leaves a very toxic metabolite in your system for another few hours. In other words, if you did a bunch of crack earlier in the day, you're much more likely to have a bad reaction, even if the earlier stuff's high has worn off.