Make Excel external references relative

Question

I'm using ClosedXML to create a really complex file referencing many external files with formulas.

The file is generated on a server, so I can't use an absolute path, but the relative path is not working properly.

Every time I try to create a relative path, Excel searches only under user documents folder, but not in the same path of the main file, where all references are.

Is there any way to specify a relative path that works properly?

Here is my code:

//In example '[filename.xlsx]sheet'!E2
string cellRef = "'[" + filename+ "]" + sheetname+ "'!E" + indexRow;    
worksheet.Cell(i, index + 4).FormulaA1 = cellRef;