1st of all, 20% is just a number thrown out. It could be 10, it could be 20. Unless you measure who knows.
I don't think it is linear. If it takes 10hp to spin the gears, 10hp to spin the driveshaft, etc why would those numbers go up as you make more power? A lot of people say it is a fixed number like the drivetrain sucks up 50hp. Some stuff goes up, some doesn't.
Corvette Z06 -1200ish rwhp