How to calculate the drag coefficient for flow past cylinder
 

Could any one tell me how can i calculate the drag coefficient for flow past cylinder using Tecplot??

i had the numerical results from my code
i triangulated them using Tecplot
then i made a circular zone
then i interpolated the data from the triangulation zone to the circular zone
then i calculate the drag force at Imax

is this procedure is correct ??

and if it is correct, why i had different results (big different) when i change the Radial (I) number or the Circumferential (J) number ??

I think there is something wrong in my procedure, could you please help me !