There is no legal requirement to have 10% ball weight or any other percentage, but legally it must not exceed the max ball weight specified by the vehicle and tow bar manufacturer.
The other legal requirements is that the laden van must not exceed the ATM, GTM and any ball maximum ball weight specified by the van manufacturer.

Note: The vast majority of van manufacturers only include the ball weight of the van in its TARE condition, why I'll never understand, but a small number now also include a maximum ball weight so that owners do not overload the Drawbar.

For Australian built vans a recommended 10% ball weight (10% of laden van) has proven to be a reasonable good starting point for safe towing, but longer/heavier vans may perform better with ball weights up around 15%. In the U.S. where some of the vans are longer than ours they recommend 15 - 20% 
The Euro manufacturers generally recommend around 6%, but their vans are generally lighter with heavy items situated over the axle and a legislated or recommended max speed of 80kph.

Personally I  err on the heavier side of 10%

Cheers, Ozjohn.

PS> Better go buy that cask of wine.