Get Office Interop Excel C# Specific Column last cell to add next row value

Hello everyone i am trying to get the Last cell for a specific column so i can add a value to it. I can get the last row used but i cant get it for a specified column.

Not all my columns are going to be the same last row value. so thats why i need it specified.
xlWorkSheets = xlWorkBook.Worksheets;
xlWorkSheet = xlWorkSheets.get_Item(“Scale_Data”);
Excel.Range xlRange = xlWorkSheet.UsedRange;
int columnCount = xlRange.Columns.Count;
int rowCount = xlRange.Rows.Count;
//Excel.Range last = xlWorkSheet.Cells.SpecialCells(Excel.XlCellType.xlCellTypeLastCell, Type.Missing);
//Excel.Range range = xlWorkSheet.get_Range(“A1”, last);
//int lastRow = range.Row;

            Excel.XlDirection goUp = Excel.XlDirection.xlUp;
            Excel.XlDirection goDown = Excel.XlDirection.xlDown;

            var colLetter = GetLastCellForColumn("6");

            /// var fempty = xlWorkSheet.Range[colLetter + rowCount].End[goDown].Row;
            Excel.Range last = xlWorkSheet.Cells.SpecialCells(Excel.XlCellType.xlCellTypeLastCell, Type.Missing);
            Excel.Range rng = xlWorkSheet.get_Range(colLetter+rowCount, last);
            int rr = rng.Row;

            //xlWorkSheet.Range(data, data,);
            //var firstEmpty = xlWorkSheet.Range["A100"].End[goDown].Row;


            /// Add New Values to their Specific Rows & Columns.
            //  Project IDrowCount + 1
            xlWorkSheet.Cells[rr, 6] = project;
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