While we are on rules of thumb for towing and distances, this one I have a belief in when it applies to towing on the ball behind a TV used for a TT.
"The rear over hang (center of rear axle to center of tow ball) of the TV should not exceed 1/2 the TV wheel base."
There was not any other clarifie'rs to this given as to how long the trailer had to be or what frontal or side area.
Another one was that the trailer weight not be larger than the TV weight. Again no mention to trailer size in relation to the TV.
Many rules of thumb fit a certain set of circumstances that make sense at the time they where developed. When the rule is given again years later, the circumstances most times are not given when the rule is delivered. As time goes by the rules stay however the conditions they are used under do change. Some stand the test of time, others can be quite outdated.
I myself would be breaking the weight rule as the TT outweighs the truck. Not by a lot, but the trailer still wins.