Good question Maria, but_ _I think Actual Effort must be expressed in hours (someone from Clarizen please correct me if I'm wrong). Generally in Project Management it is a commonly accepted to use hours instead of any other units and Clarizen sticks to this rule. What you can do is set a day work (e.g. 8h) within organizational calendar and report 8h (that would be 1 day) and so you can get index value correct.
If there is any way to do that - please share your knowledge.