
The column index number is a relative number based on the range you specify. For example, if your range is D1:G27, then column F would be specified as 3.
If your range is so wide that figuring out the relative column is too complicated, the you could specify the third argument with a simple formula. In my example, you would use
VLOOKUP(3.14159,D1:G27,CODE("F")-CODE("D")+1)
If you really want to see column numbers instead of letters, go to File->Options->Formulas and select the "R1C1 reference style" option