[mattbrewsbytes]

I used to use 25 hours out of 40 in burndown calculations, so 5 hours a day. This was then used against task lists that had estimates in hours of 1,4,8,16 increments. If something needed a re-estimate after work began that was taken into calculation.

That was with a fairly low amount of BS. If there a lot of meetings then lower it. If you work in an open plan work space with people not directly contributing to the same effort, lower it some more.