The simple answer is - it is not.  The windfall tax bite is really nasty, here is the calculation if we use all the same assumptions:

"Example...

Lets say the base is $ 850 and gold goes to $ 1850 and it costs you $400 an oz to produce the gold... Your WPT per ounce would be $ 1000 x 70% = $ 700" OK let's go.....

                  W/O WFT     With WFT    Fav./(Unfav.)

Revenue     $1,850         $1,850          $ $  0

Expense         (400)            (400)                0

WFT                     0              (700)          (700)

IBIT             $1,450          $   750           (700)

Tax (40%)       (580)            (300)            280

Net income   $   870         $   450          $(420)

The windfall tax is actually just a governemt slap on expense (based on the way it is applied), calling it a tax is confusing.  The other thing is the WFT is levied on the price of gold, so there is unlikely any expense related to it, because regardless of how much gold sells for, there is little correleation to the extraction cost per se.   The 40% tax rate is just an illustration, as you pointed out, actual composite tax rates in Ecuador are lower. 

p.s for what is worth I am a retired accountant who has done a bit of work on financial modelling.