Other articles on using a 6.5 hour work day for software dev projects

I haven't heard of derating a day from 8 to 6 1/2 hours, I have usually derated day estimates by 15% to account for vacation, holiday, and sick leave. A lot of it, however, depends upon where the baseline values are coming from.

I would only accept a value in hours if the number was 4 hours or less. This will be true working hours, ignore various overhead activities. Any time I hear 8 hours or 40 hours, I presume the estimate is actually in days or weeks and has an informal overlay for daily activities.

From past experience, I would avoid trying to derate worked hours into "productive work" hours. It simply leads to too much confusion. It doesn't matter whether the derating gives the "right" answer, if the method confuses.