I figured out part of the answer.

In the Export function, I added the line range[2, 7].EntireColumn.NumberFormat="#.##%;. This now displays the calculated value as Percentage on export. I still have a minor issue where it addes the decimal point after a resulting value of 100%, so I still need to work on that one.


            for (var b = 0; b < dt.Columns.Count; b++)
            {
                xlWorkSheet.Cells[1, b + 1] = dt.Columns.ColumnName;
                range = xlWorkSheet.Cells[1, b + 1];
                range.Interior.ColorIndex = 15;
                range.Font.Bold = true;
                //range = (Excel.Range)xlWorkSheet.Cells[1, 7];
                range[2, 7].EntireColumn.NumberFormat = "#.##%";
            }