When you have a sequence counter use =Chr(Counter+64) as ControlSource.
If you don't have on, you can simply generate one by modifying your
RecordSource of your report. At a column SequenceBase: 1. Create a
running sum control based on that field. Use that control in the first
mentioned function: =Chr(runningSumCtl.Value + 64).
First, add a hidden text box (named txtLineNum) to the
detail section, Set its control source expression to =1 and
set its RunningSum property to Over Group.
Then you can use another text box with the expression:
=Chr(Asc("A") -1 + txtLineNum)