The tip of the day in Managing Projects is about Efficiency vs Availability

Availability is how much people are accessible for work. If a person spend 20% of their time on leave, vacation, training, and other non-work effort, and there are about 160 hours in a month (after holidays), they are only 80% availability or 128 hours.

Efficiency answer the question of how much can they get done in an eight hour day. If you take into account meetings, e-mail, side conversations,… the best expectation is about 70% to 80% efficient.