Writing from RPGLE to Excel - using a formula on one worksheet to reference data on another worksheet

I'm using Scott's spreadsheet utility from SystemiNetwork (thanks Scott!)
and am having problems when trying to write to a second worksheet.  I'm
trying to replace a manual process where data is copied into several
worksheets (one for each company), then the data is summarized for all
companies on another worksheet.  There can be different amounts of detail
for each run, so a static formula pointing to ranges on the other worksheets
can't be used for the summary worksheet (hm, named ranges might be a
possibility).

I have the detail along with subtotals for that worksheet working fine in
the first worksheet.  Next, I open a second worksheet and try to write a
formula referencing the first worksheet.  The formula is written to the
first worksheet instead.  I suspect I'm missing something simple here, such
as needing to close the first worksheet first.  Debug does show the second
worksheet as being my "sheet".

Here are some code snippets to show what I'm trying to do.
      // Set Begin point for cleanup, up to 100 objects.)
      hssf_begin_object_group(100);

      // Load a copy of the workbook.
      CurBook = BookIn;
      Book = hssf_open(%Trim(CurBook));

      <small snip - style setup>

      // Open the spreadsheet within the workbook. <== first worksheet
       sheet = hssf_getSheet(book: 'Company One');

       ExSr $CellDetail; <== this writes out detail records and summarizes
transaction types

      // Open the spreadsheet within the workbook.  <== second worksheet
       sheet = hssf_getSheet(book: 'Summary');

      ExSr $CellSummary;  <== this writes my summary formula to the first
worksheet, not to the second worksheet

I've tried saving the file to disk, then reloading it before opening the
second worksheet, but the formula still appeared on my first worksheet.  For
this, I inserted the following after the ExSr $CellDetail;  and before the
sheet = hssf_getSheet(book: 'Summary');
         CurBook = BookOut;
         hssf_save(Book: %Trim(CurBook));

         CurBook = BookIn;
      // Book = hssf_open(%Trim(CurBook));
and the first worksheet still has the formula that should be on the second
worksheet.
When I remember to leave off the // on the last bit of code, the first
worksheet does not have any data in it at all.

Hopefully this is enough code to let you show me what silly thing I'm
missing.

Thanks.
Mark Plank