@greggyb Thanks for putting this together so succinctly. In applying your solution I found that your first expression should actually be:
TotalQuantity = SUM( 'Table'[Quantity] ) % Total = [TotalQuantity] / CALCULATE( [TotalQuantity] ,ALL( 'Table' ) )
Thanks so much for this, it was surprisingly hard to find a concise write-up on how to do this. I think this about the fifth time I've used one of your DAX approaches, so your contributions are very much appreciated.