There is no "80% rule" that anyone can document. There's an old article about towing in the Rockies at high altitude that notes 80% of the Combined Gross Vehicle Weight Rating worked well. This is different -- and much lower -- than 80% of the maximum towing capacity.
You can pull your maximum towing capacity but you won't like it. Trailer weight must be reduced by all weights in the truck itself. The simplest rule is the "1000 Pound Rule" which even I can understand. Have 1000 pounds more towing capacity than the trailer weighs (this accounts for cargo and passengers). Conversely limit trailer weight to 1000 pounds less than maximum towing capacity. In your case that's a 5200 pound trailer weight. But... I towed a 5000 pound Roo 23SS with my 5500 pound max rated Explorer for two seasons. Didn't line the feeling of flogging the poor beast and upgraded the truck. The Explorer? Ran another 6 years and 50,000 miles before it was time to trade it.
-- Chuck
