From a thread at my favourite shave forum:

The percentage is around 35% mane and 65% tail but it is not always used in the exact same percentage in every production – it depends on the raw material received each time. The softness is obtained thanks to the mane and the backbone thanks to the tail.

So, speaking logically, a 50-50 mix as some of the Vie Long brushes have will be softer and with less backbone. Good information for those who find Vie Long’s regular brushes to be too stiff and scratchy.