Agreed, @DannyP. I've never liked the 20% rule, as it assumes that every drivetrain is equally inefficient. It's just the number everybody uses.
Turbo hyrdamatic 400 automatic running through a 3000 stall torque converter and a 12 bolt rear end? 20%. 4-speed VW transaxle? 20%.
Its only value is to provide a conservative estimate on crank horsepower, rather than a hopeful one, and that's rather in short supply in a sea of hype.