It is simple to save default for Writer docs. The same method to default save Calc sheets does not seem to be available. Suggest that when in Calc the selection of default save should change to the same selection method as for writer docs. eg save default to save as .xls
NEVER use a foreign file format for ongoing work and, thus, NEVER use a foreign file format as a default save format. It is a straight way to data loss. Use only ODF and only after you have finalized your document, export it to something else.
It’s at the same place, Tools → Options → Load/Save → General, just instead of Document type Text document select Spreadsheet and then the corresponding Always save as value.
Thank you the path to setting the default save is now understood.