AFAIK, the frictional losses from the drivetrain are proportional to the power (load) being transmitted through them. There are also "fixed" losses that remain constant, no matter what the power/torque levels are. That said, there is no way a standard percentage value can be used for an accurate calculation, especially across different vehicles/engines/drivetrains. I think it's more of a rough guideline at best, and I've heard anywhere from 10-20% used as the "percentage of loss".