G'day

Something to try: Assume

A B C D

1 Sheet Select Sheet Formula
2 Name Sheet Total in Column C
3
4 Sheet1 X 1000
=IF(B4="","",Sheet1!$B$1)
5 Sheet2
=IF(B5="","",Sheet2!$B$1)
6 Sheet3
=IF(B6="","",Sheet3!$B$1)
7 Sheet4 X 4000
=IF(B7="","",Sheet4!$B$1)

Total of Selected Sheets 5000 =SUMIF(B4:B7,"X",C4:C7)

Change the cell references to suit.

You could have the user select in two ways:

1. They have to manually type in "X"
2. Create a dropdown list with the "X" in it using data validation/name
range.

I would then consider creating a macro and placing it on the sheet to delete
the selected "X" rather than doing it the manually.

HTH
Mark.