RowColumnConversion.cs

var serials = new[]
            {
                "00001", "00002", "00003", "00004", "00005",
                "00006", "00007", "00008", "00009", "00010",
                "00011", "00012", "00013", "00014", "00015",
            };

            const int rows = 5;
            var columns = (int)Math.Ceiling((double)serials.Length / rows);
            var result = string.Empty;

            var packageSerials = serials
                .Select((value, index) => new { value, index = index / rows })
                .GroupBy(x => x.index)
                .Select(g => g.Select(x => x.value).ToArray()).ToList();

            for (var index = 0; index <= rows; index++)
            {
                result += packageSerials
                    .Select(array => array.Length > index ? array[index] : string.Empty)
                    .Aggregate(string.Empty, (accumulator, currentSerial) =>
                    {
                        if (string.IsNullOrEmpty(currentSerial)) return accumulator + currentSerial;

                        var row = index + 1;
                        var column = packageSerials.FindIndex(array => array[index] == currentSerial) + 1;

                        return $"{accumulator}R{row}C{column}={currentSerial};";
                    });
            }

            
            Console.WriteLine(result);
            Console.ReadLine();