So have any of you fancy math folk been able to determine exactly how this skill works?

It says +20% CHR, however there's no clear indication if that's a flat +20 to whatever your existing chance is, or if it's 20% of that chance i.e. if your current crit rate is only 10%, then it rises to 12% with the skill active (I don't think it's the latter but confirmation would be nice).