Auto fill formula with offset

Hi everyone,

I am currently making a module to automatically add columns in a sheet.
I would like to insert a new column and fill it on ~1000 lines with the formula =Bx/Ax if this column is C. The problem is that I don't know if the column is gonna be C or D or Z, so I guess I have to use offset to make something like =cell-1/cell-2.
A and B would be filled manually.

I'm new in VBA, I can't get it right to have the formula and then fill in all the column with it, can someone help me ?
Enclosed is a example of the result I'd like.

Thank you,

Jules