Windfall Levy on condensate

[sirjee]

i want to calculate windfall levy on crude oil (condensate). I got a formula from a brochure issued by ministry of petroleum and natural resources i.e
0.5 X (net production - royalty) X (market value - base price)
whenever i use this formula i experience a material difference between amount charged by the operator of the field and the amount calculated by me. Is there any other calculation involved which I am missing?