Critical hit rate.
Which agrees with what I described in my 6th paragraph.
Its possible, since I haven't worked with the skill much, that the Additional Critical Damage is actually Additional Critical Chance. That's how regular Counterattack works after all. That's also how Lance Counter works, so I find the "Critical Damage" to be very odd. If you get a chance, try to see if the rate of Criticals from Raccoons seem to match the rate the skill's "Critical Damage" is supposed to be at.
I was just using the event bonuses to finish of training all the Rank 1 fighter skills to mastery level on my elf. To finish off the "Get a Critcal Hit with Counter Punch" part of rank 1 Counter Punch I took off all armour to eliminate protection and jumped into Dorren's Request Hard Mode. As above, used the green switch to get crag cows and shadow shires. Loaded counter punch 56 times and 28 times I got an increment to the training count. So exactly 50%.
When the stats page talks about critical hit rate is states "However, regardless of the player's critical and the target's protection, the chance to score a critical hit is capped at 30%." My elf is sufficiently advanced to hit that cap. And on the Counter Punch page, it claims Rank 1 Counter Punch gives a 20% boost to what is described as "Additional Critical Damage", but multiple people have suggested that it is really "Additional Critical Probability".
30% + 20% matches the observed rate of 50%.
And after I had done that I realised that while I had removed all armour, I'd forgotten to remove a title, so in the character window my protection was showing as "5 (0)". So 5 protection by zero base protection.
So it looks like at Rank 1 Counter Punch, with minimal protection and reasonably powerful monsters we can get 50% of our Counter Punches to score as critical. But with higher protection and weaker mobs like raccoons or dire wolves the rate is way lower. (I never formally collected data, but it felt much lower than 10%. It was tediously slow training!)
Anyhow, that's Counter Punch fully mastered now on all 3 of my characters, so I won't be generating any more data on this issue.