Biff wrote...
The workbook, New Monthly Invoices June 05.xls, *MUST* be open for this
to work.

And the formulas will crash as soon as the other workbook is closed and a
calc occurs in the active wb!

Users should be made aware of this significant drawback!
....

In Excel 2002, if the other workbook isn't open, my formula returns
#REF!. While I wasn't explicit about how the formula wouldn't work, I
was clear than it wouldn't if the other file weren't open.

By 'the formulas will crash', did you mean something other than
returning a #REF! error?