20 percent rule ??
Thats something from the past when our LDT truck engines had some puny hp/torque numbers/poor brake systems and smallish radiators....no locking tranny torque converters. Good advise back then.
Its not needed today with the newer gen diesels and high hp v8 gazz engines.
Legal issues ??
GVWR nor a trucks GVWR based payloads have any legal status in determining a overloaded truck or other silly opinions like you can be sued for exceeding a truck mfg GVWR or its payload.
I have the same truck as yours.
It a '03 2wd 2500 Dodge/Cummins 305 hp/555 torque HO quad cab NV5600 tranny 3.73 gears and has a 6000 rawr with LT265/70-17 E tires.
My truck empty rear axle weighs 2840 lbs scaled weight. This leaves my truck with around 3160 lb for its legal payload.
I pull a 32' 11400 lb 5th wheel trailer. My pin weight various 200-300 lbs depending on the length of the trip and where I load things in the trucks bed and front of the trailer. My truck has 5400-5600 lb on the rear axle when its all loaded and hooked up.
Dodge 2500/3500 srw and DRW trucks from that era all had the same 11" AAM rear axle rated well over 10000 lbs.
RAWR is the lessor of the vehicle mfg wheel...tire...rear suspension and complete rear axle assy.
All 3/4 ton trucks with gas or diesel engines have about the same rear axle payload as they will scale about the same from the factory. Of course adding in the bed fuel tanks....heavy weight hitches....and other junk we carry in the bed lessons the actual payload.
You have the truck so drop by a set of local scales and weigh front and rear axles separately. That way you know exact weighs your truck can safely carry while pulling any trailer.