These guides are not worth the paper upon which they are printed. There are way too many variables to be accurate. You could have many capacities for the dually, because every model has a different capacity. Examples: reg cab, ex cab, crew cab, 2 wd, 4 wd...you can have any combination of the first three with the last two. All have different capacities due to different weights.
Using my truck as an example, my 5th wheel towing capacity is 16,400. If I had gotten the ex cab rather than the cc, it would be 16,600 because the ec is lighter. Those guides will give you ball park figures, but that's all.