Hi

This should automatically be taken care of if you INSERT rows. When
inserting rows and you used to sum to 15 you can insert two rows at 15
(but unfortunately your number in 15 will shift down).

The alternative is to make your sum until 16 even though your last
number is in 15. Then for future updates, you can insert two rows in 16
and the last number will stay in 15 but your formula would include two
more rows for your extra data.