Don,
TW can range from on average about 12% as mentioned, to possibly as high as 15%. Some trailers like more, some tow just fine with a little less.
Using 12% based on the trailers gvwr is a good start, since chances are you will not be loading the trailer to 100% of the gvwr.
Also as mentioned, just watch what you load in the bed of the truck once you figure out what your tw (including your wd hitch) is, and how much the passengers weigh.
When you get the trailer, and get it loaded, load the truck and trailer and hit the local CAT Scale. Weigh 3 times total:
1: Truck and trailer ready to roll down the road
2: Truck and trailer, but with the wd bars unlatched (wd bars in the bed of the truck)
3: Truck only, leave hitch in and wd bars in the bed.
Post up your numbers and we will be able to figure out what your tw is.
As for towing, just take your time, don't cut the corners, and as mentioned don't be afraid to practice in a local parking lot! Consider setting up some cones and practice seeing how the trailer tracks when making a 90* turn near them, but don't run over the cones!
