I was being a little sarcastic in my previous post.

My point being that I don't see how a flexible winfall tax is better than the standard taxes that most governments use. Other than for the government to save face on a tax measure they previously announced I see no reason to reinvent the wheel here and over complicate what should be a straight forward measure. 

That being said if this solves the political puzzle here and gets us a reasonable deal than I am all for it.