There are tow issues. Of course one is the CG and percent loading on the tongue and the other is the polar moment of inertia.
Polar moment is the concentration or distribution of weight from one end to the other.
Weight concentrated at the axle has a low polar moment and distributed to the ends has a high polar moment.
The high polar moment takes more to start an oscillation, but more to stop it as well.
Since the trigger is usually from some outside source the damping must be from the towing system itself.
A longer tongue helps damp the oscillations and the lower polar moment helps as well.
Also as Norm stated the stiffer sidewalls also helps both prevent and control the oscillations.
If the polar moment were very low then the tongue weight could also be very low as well, but zero is not possible so 8% it is.
It is likely that when the rule of thumb was developed mos were towing with old pickups that were very light
in the rear end and the extra weight helped the handling, but today the suspensions and weight distribution is better.
Rules of thumb had their origins somewhere, but that doesn't mean that they apply in every case, however unless the TV rear suspension is overloaded or the handling is degraded to where the handling changes from understeer to over steer below .3 G side loading there is safety in error in this direction.
Hard to question erring in the direction of probable safety.
Personally I agree with Norm on this with a modern front wheel drive car or van. If you have a nine passenger lifted van with long overhang and soft tires it won't be safe anyway so higher tongue weight might help.
Since I failed to answer the question of threshold the correct answer id included above.
The TV and trailer must not change from understeer to oversteer at less than .3 G of side loading. And the system must be damped to where oscillations decay withing two oscillations. Every TV - trailer combination has a critical speed where the damping has decreased to zero and beyond that speed the combination will be unstable.
I have noticed that with my VW JSW the combination is well damped and very stable, probably because of the IRS and negative camber with toe-in at the rear.