What %-to-hit-head chance is required to make head hunter worth it in your opinion?

I found a great longaxe with +19%-to-hit-head and apparently I previously assumed flail %-to-hit-head to be higher cause I wasnt even sure if head hunter would be worth it on the bro using it. Now that I checked flail hit-to-head to be just 15% I am wondering what the threshhold is for head hunter to be usefull.