The best way to create reusable formulas in your workbooks is to add your table to the data model (which creates an in-memory tabular cube) and write measures using the DAX formula language. This has the added benefit that a single formula can be written to aggregate data at different levels, for example, a sum can be calculated over days, months, or years. This will only allow you to share formulas in a single workbook. The data model in Excel is powerful and under utilized.