We don't know in advance whether we'll need an Xls or an Xlsx workbook. For example, we might end up needing more columns or rows than fit in an Xls. According to some other posts and correspondance, I changed:
I was under the impression that I could decide at save-time which format to use. In this case, I am always saving as Excel97To2003.
In the attached files, Portfolio Balancing Good was made with the first call and Portfolio Balancing Bad was made with the second call. There were no other changes to the code. You can see that the Bad file has numerous #NAME? errors. Apparently, function names are not being correctly saved in the XLS. If you look at cell Controller!B6, for example, you see that the call to sum appears in lowercase. If you F2/Enter in that cell, the #NAME? error is corrected and SUM now appears in uppercase.
While we can live with making only smaller files for now, we anxiously await the ability to have our sheets exceed XLS limits.