Best way to change a cell value when another cell value changes

Question

Please help identify the most efficient way to update a designated cell (Defined Name = <worksheetname>active_row) to reflect when a cell in column G changes to a non-null value.

• there are multiple worksheets in the workbook having this identical need
• I use a Defined Name for the last cell of column G to be evaluated (<worksheetname>_end_row)
• the range of cells for all worksheets is $G7:<worksheetname>_end_row

Is this best accomplished with a macro or VBA?

Can you recommend any specific resources illustrating this technique? (I have looked and have not yet discovered applicable on-line examples).

Thank you in advance.