Define a calculation field as:
Code ( LetterField )
Then a summary field, sMaxLetterCode, can defined as the maximum of this calculation field.
And this expression will compute the next letter in the series:
Char ( sMaxLetterCode + 1 )
This will only work if you have a found set of all records. A similar method, however can produce the same results via a self join relationship and then the state of the current found set is not a factor.
Thank you. It works perfect.