You probably should define a field with an auto-entered serial number instead of adding the text "@@" to your layout. The @@ text will report the current number of the record as it is currently sorted in the found set. The same record will show different values just by sorting the record or performing a find to pull up different groups of records. This is not likely what you want.
To include leading zeroes, just add the leading zeroes to the "next value" box on the field options auto-enter tab. (Open Manage | Database | Fields and double-click the field.)