I will state now that I fully believe it is sensible to keep the weight of any trailer and especially caravans as small as possible to minimise the onset of towing instability. Whilst I don't believe the caravan industries advice was based on solid or repeatable evidence, it is a starting point.
Weights are important, and there are legal limits (GTW GVW and axle loads) which must not be exceeded under any circumstances. You must also be aware of the limits your driving licence may impose, so check.
The UK caravan industry advice is not legally enforceable but it does indicate that towing caravans is not something to be done without a lot of forethought, and care to minimise possible loss of control.
Speed is always the single biggest factor when loss of control occurs, and weight ratios are a factor but so are loading practices, and vehicle condition.
The faster you tow the more likely you are to start to notice the onset of instability. The most important action you can take is to slow down gently. The actual speed where instability may be noticed is variable and affected by driving style, and the prevailing road and weather conditions. There is no automatic right that you must be able to drive as fast as the permitted speed limits. If your outfit is unsettled you must slow down.
Sometimes all it may need is to rearrange the loading of the trailer, but be aware of the mechanical limits of the tow hitches nose load limits.
A good driver will be sufficiently aware of how the outfit feels and should detect the onset and reduce speed before it become uncontrolled.
There is no mechanical magic that makes 85% or even 100% ratio's significant, but they are values that can readily calculated, even if the way they are specified is not particularly accurate. It is a guide, and starting point.