From the by you provided code snippet I miss the essential part from the blog you referred to.
As stated in that blog OpenXML can only be used for creating/reading/updating and deleting the storage format for OpenXML compatible applications. You don't get the actual application engine. To achieve what you want you either need to install Excel on your server so you can leverage Microsoft.Office.Interop.Excel namespace and it's Application
COM interface or rely on the end-users Excel application to do the recalculation when openening the updated excel sheet.
code copied and text adapted from this blog
var excelApp = new Microsoft.Office.Interop.Excel.Application();
Workbook workbook = excelApp.Workbooks.Open(Path.GetFullPath(_filePath));
workbook.Close(true);
excelApp.Quit();
Please do realize that if you need this to run on a server (for example because you need the calculated results in a webpage) you might run into serious performance trouble, memory, I/O and CPU wise. In that case I would rather implement the calculation rules/formulas in C#.