Add calculated field to a powerpivot table?

Hello, I am struggeling to get a weighted average discount % in my (power)pivot table for a customer with various order lines. See attached document. Does anyone know how to get this done?
Thanks a lot!
Emile

Hi - unfortunately I don't see your attachment

Apologies, apparently I got an error in uploading. I now inclused it in a previous post and now it's working. Thanks again!

Status update: issue is solved, thanks all!