Or quit thinking of them as resources or machines that were advertised to you as 100% efficient for 8 hours 5 days a week and realize that the 2 hours in a given 8 hour day spent reading HN or browsing YouTube _are_ part of the 100%. No one can walk into an office and be at 100% efficiency within microseconds. Windup and spin down time is part of the cost of running an organization. Not only is there windup/spindown at 9AM and 5PM, but at 12PM and 1PM, and probably at around 3 or 4PM in addition.
If someone is only putting in 2 hours of efficient work a day, then maybe you should start thinking about shuffling them within or out of the organization depending on the position and level they're filling, but asking about 10% inefficiency due to "missing bodies" is equivalent to complaining about getting shorted a penny from the barista. It's literally pocket change, get over it.
If you ask them, most developers will have something about the current codebase they would like to improve, or a 20%-like project that might improve the products. You can have them do this during times of over-availability.
