I'm trying to write a python script that will calculate accurate percent complete for Epics.

[Dave White]

I'm writing a python script to walk through the child stories of the Epic and calculate the percentage done based on the "statuses" of the child stories.    Jira does this on detail pages however, it does it based on "Done" stories in the Epic, and doesn't take into account the In Progress or Testing ones.   The script works.   

However, in trying to update a new field (Completion Percent) in the Epic, it's throwing this error:   

response text = {"errorMessages":[],"errors":{"Completion Percent":"Field 'Completion Percent' cannot be set. It is not on the appropriate screen, or unknown."}}

It IS on the editing screens and is editable.    Could use some insight on making this work. 

thanks, Dave