Item Master Lead Times for purchased items

We are in the process of reviewing the lead time attributes for purchased items, specifically the preprocessing, processing and postprocessing times.  We need to determine if if we should be deducting non-workdays from the totals.  I entered an SR with Oracle and they pointed me to a document which only discussed the calendar for manufactured (make) items and did not seem to apply to our purchased (buy) items. 

Does anyone know whether we should be basing our calculations on calendar days or work-days?  It will make a big difference in settings.

Thanks

Jeanne