Depending on what is meant by “for a whole column”, @hjek’s answer may apply; or, as I suspect (based by your “I would like to have a fourth column that has the value of the formula for each line”), you could mean simply how to copy the formula to all cells in a column.
Then, you can enter the formula to, say, K23 (as you show); then copy the cell (not the text in cell!) - single-click to another cell first, then single-click back to K23, making sure that you are not editing the formula, then
C; then select all the cells in the column (or - preferrable - a subrange adjacent to your data) - by clicking with mouse on the column’s header, or by putting cursor to last cell and using
↑, or by entering the range (K:K or K1:K123) into Calc’s Name Box at top left corner of the sheet; then paste:
By the way: the formula
=SUM(J23/(G23-H23)) is equal to this simplified formula: