Might be a little late on this post but....
Just got back from sitting my OZ ATPL and in the Systems/Aerody exam casa seemed to think that you add half the the headwind to you Vref up to a max of Vref + 15 and then on top of that half your gust to a max of Vref + 20. eg.
W 100/20G30 = 10 + 5 = Vref + 15
But thats only CASA.....