Its based on the d3 result;
You roll a d6, divide it by 2 and round up as needed.
My preachers just grew a second head last night as he rolled double 6's on a high-level sermon and failed several of the corruption hit saves.
I will be trying to get to a big town to increase the odds of finding a doctor that can cut that second head off. This raises moral questions, but for now, I am fine with the head murder.
PS: Shall I rename him "Zaphod?"