document.WorkbookPart.Workbook.CalculationProperties.ForceFullCalculation = true;
document.WorkbookPart.Workbook.CalculationProperties.FullCalculationOnLoad = true;
As per MSDN:
Use the CellFormula (<f>) element to define the formula text. Formulas can contain mathematical expressions that include a wide range of predefined functions. The CellValue (<v>) element, stores the cached formula value based on the last time the formula was calculated.