Help - How do i devide in a report?

Content

Hi,

I'm trying to get add average response time to my chat transcripts report, I cant find a field for this so think I'm going to have to make a customer field. I have the agent total response time (chats.agent_resp_secs) and the agent number of messages sent (chats.agent_num_msgs) so I logically though adding an excel type formula of =chats.agent_resp_secs/chats.agent_num_msgs  would be suitable to give me the average response time but it wont accept the formula.

What am I missing?

Thanks