Macro to apply a formula in a column if there are values in another column

Friends,

I have around 50 columns in a sheet.

First column (A) is having serial number. I need to apply a formula [i.e. =SUM(AF4-AB4) in column AT4:AT ]

For example:

If there are values in column A4 to A38, then the above formula to be applied in column AT4:AT38.

If there are values in column A4 to A140, then the above formula to be applied in column AT4:AT140.

If there are values in column A4 to A263, then the above formula to be applied in column AT4:AT263.

It will be more better, if the result of those formulas are converted to values.

Request your help.