Paul,

this is a BeforeSave, not BeforeCode code you posted. (almost) Exactly
the same code can be pasted inside

Private Sub Workbook_BeforeClose(Cancel As Boolean)
....
End Sub

I say (almost) b/c I believe you could replace ActiveSheet with a
specific sheet. This way, before closing the workbook the macro will
always check the required cells regardless of which workbook is active.
I am pasting my recommended code below, in which the OP can replace

Sheets("Sheet1")

with whatever is the name of the form sheet inside the quotes.

HTH
Kostis Vezerides