There certainly is a way that you can access your formula calculator directly from your desktop. What you may do is to actually turn the Excel Sheet into your Desktop background, so that you may then work on and edit it directly.
In order to do so, you should first access the "File" menu in your Excel Spreadsheet. Then, you should choose "Save as Web Page," after which, you should select the "Add interactivity" option. After these preliminary steps, you may simply set the resulting saved document as your desktop background, and work on it as you would on a regular spreadsheet.