|One way I can think of is to create a new sheet, call it Totals|
In Cell A1 enter the TEXT number 001, it must be formatted as text else you lose the leading zeros.
In Cell A2 enter the TEXT number 002.
Hightlight cells A1 & A2 and drag down 300 rows.
You should now have column A filled with the TEXT numbers 001 - 300
In cell B1 enter the text: C15
In cell C1 enter the formula: =INDIRECT(A1&"!"&$B$1)
drag the formula down 300 rows.
You can now either do a simple =SUM(c1:c300) on row 301 or you can do a running total using column D