When they say "divide by .13" above, I find it MUCH easier to simply MULTIPLY BY SEVEN.
Load up, weigh your truck, then take the remaining payload and multiply it by 7 to get a good estimate of how much you can tow.
There is a little "fudge factor" built into that 7, because the trailer is ALWAYS heavier than you think it is, especially if you don't weigh it.
One thing I do recommend is, load everything you possibly can into the trailer. Cooler of drinks? Trailer. Bikes? Make 'em fit in the trailer. You can tow 7 times as much as you can haul. You want that truck bed EMPTY when you're talking about something with a limited payload capacity like an Avalanche where you're only starting out with 1350lbs to begin with.