Hi Hugh,
If you feel you have poorly designed codes in GL, AP or other applications, the auto-fill won't be the answer. Rather, you probably should be looking at the
Toolkit for Adagio, which is an add-on product that will allow you to re-code your Accounts, Vendors, and many other records.
After your codes are configured in a consistent way, with all having the same length, then the auto-fill will be a useful addition. And you can specify the format that new codes must follow so that your coding does not go awry in the future.