In North America, trailer tongue weights are typically 10% or so of trailer weight although there is certainly variation and it can often be as high as 15% as you mention. That's likely why the manual reads the way it does...and yes, it's unfortunate that they don't spell it out more clearly that it's about tongue weight relative to WDH. Trailers in Europe tend to balance differently and have much lower tongue weights so they can be towed with more modest vehicles, albeit typically with speed restrictions that don't exist here. WDH doesn't typically get used there because trailer brakes have to be inertial/surge brakes and not dependent on the tow vehicle. Different system over there...