Use a Fixed Cell in a Formula which changes based on the row of a column

Dear Experts,

I have a situation where i want to use a fixed cell in column A in a formula in Column B. However, i want to change the reference cell in Column A when i reach row 10+1 in Column B. I need to automate it as it has to be repeated for thousands of lines. I have illustrated it in few pictures.

Any ideas would be great.

Thanks