Help with subtracting numbers in columns

I have a spreadsheet where I have entered the formula =SUM(I1)-L1 in Column J (this formula is then copied down Column J to match the number of rows in Column I & L).

What I want to do is create VBA code that will automatically subtract Column L from Column I with the result in J, but I'm unsure how to do this when the number of rows is different from spreadsheet to spreadsheet. I then need a subtotal of Column J to show at the end of the column.

I've attached a spreadsheet showing what it looks like after the calculation.

Any help would be much appreciated!

What do you expect from VBA in which you would not get from a formula?
In other words, what a formula can not do while VBA can do.
It is not clear from your sample.

See attached for spreadsheet before the calculation..

Thanks so much. That worked perfectly. I appreciate you taking the time to help me out.
Have a great day!