Try this one from raybaudi...
and wrap your expression inside it (or use two fields...1. total number of days, 2. number of working days: using #1 as the field inside raybaudi's expression.)
Could you help to clarify this a bit do I need to write another calculation? Could you give me a brakdown how this works or how I would write it? I'm a little lost still a rookie when it comes to FM.