I also wonder for the more seasoned martial artists out there how much control do you actually have to not go a little too far?
It really does all depend on how surprised or tense you are (by 'tense' here I mean that alert state you are in when you are expecting trouble but don't know where from). If you've seen a situation building and are aware of the potential for for violence then you can gauge your response much more finely.
If someone grabs you out of the blue then there is a large degree of truth to the fact that why we train so hard is to drill things in below the level of conscious thought. You literally react on brain-stem reflex and your best hope of dialing it down is trying to stop the technique before it finishes e.g. open your fist, drop your shoulder, lift a foot, anything to take the power out of a strike.