IMHO Ford states it well in their towing guides, albeit in asterisk fine print:
Ford Towing Guide wrote:
Trailer tongue (trailer king pin for 5th-wheel towing) load weight should be 10-15% (15-25% for 5th-wheel towing) of total loaded trailer weight. Make sure vehicle payload (reduce by option weight) will accommodate trailer tongue (trailer king pin for 5th-wheel towing) load weight and weight of passengers and cargo added to towing vehicle. Addition of trailer tongue (trailer king pin for 5th-wheel towing) load weight and weight of passengers and cargo cannot cause vehicle weights to exceed rear GAWR or GVWR. These ratings can be found on the vehicle Safety Compliance Certification Label
