SetGT/ReadPE method Exception

I have a account memo file that is uniquely keyed by account# and memo 
sequence number. When I want to add a record to an account, I do the 
following:

- SetGT using the account number
- ReadPE using the account number
- If a record was found, add one to the sequnece number
- If a record was not found, use 1 as the sequence number
- Write the memo with the error extender
- If there was a write error, repeat the loop (somebody added a memo during 
the above steps)

So this works beautifully until today -- The account being added to was the 
last in the memo file. The SETGT fails, 1 is used, and the write fails 
repeatedly. I am trying to find an efficient way to determine what memo 
number to add. Perhaps I have to resort to SETLL/READE to the end, add one 
to the seq#. Any ideas?