Based on my Comment here is one way to get what you want done:
Start byt selecting any cell in your range and Press Ctrl + T
This will give you this pop up:
make sure the Where is your table text is correct and click ok you will now have:
Now If you add a column header in D it will automatically be added to the table all the way to the last row:
Now If you enter a formula into this column:
After you enter it, the formula will be auto filled all the way to last row:
Now if you add a new row at the next row under your table:
Once entered it will be resized to the width of your table and all columns with formulas will be added also:
Hope this solves your problem!