Work with the example
How many pages belong to a chosen three-period window? B5:G5 have six complete period counts. I5 supplies the start-column index and I6 the length. I7 calculates the end index; K5 checks that both controls are numeric positive whole numbers within the table.
| Periods | Pages |
|---|---|
| Oct,Nov,Dec,Jan,Feb,Mar | 4,6,8,10,12,14 |
| Start 2, length 3 | Nov–Jan total 24 |
Edit the working cells
After validation, K5 uses SUM(INDEX(B5:G5;1;I5):INDEX(B5:G5;1;I7)). These INDEX endpoints form a live range. Change December D5 from 8 to 9: the total becomes 25. Fresh restore 8, start 2/length 2:14. Start 5/length 3 gives Window outside table; zero or fractional controls are refused, and text "2" gives Window missing.
Check the failure case
A fixed SUM(C5:E5) keeps summing Nov–Jan when the user chooses a different valid length. Changing a control cell is useful only when the actual formula range follows it. This complete row makes no missing-period imputation.
Why is start 5/length 3 invalid for six columns?
Show the worked answer
Its end index is 7, beyond the sixth supplied period.