I need to sum up a column of noncontiguous cells.
(eg. G3+G6+G9+G12); with the result being presented at
the bottom of the column.

However, whenever I insert rows in between the data and
the row that contains the total sum, I need to manually adjust the summation
formula.(eg. formula above + G15+G18+G21).
On the other hand, when I delete the rows in between the data and row that
contains the total sum, a reference error pops up. (so I need to manually
adjust my formula again)

I have at least 50 columns to perform this summation for. Is there any sort
of formula that helps to minimize the manual labour?

Thanks you in advance!