Hi

I would go for your second option:

Or replace (#n/a) with a zero?

In excel 2007 use an "IfError" formula in the cells to sum like this:

=IfError(A1/B1,0)

Which will return a the result of A1/B1 or 0 if the formula return an error.

In previous versions use this:

=IF(ISERROR(A1/B1),0,A1/B1)

Hopes this helps.
....
Per