In ADT 2005 Viewport & drawing scale can be selected via the drawing status bar. There is a good selection and also custom but if that’s not enough Chris Yanchar mentioned in a newsgroup post:
"The list of scales is much more extensive, and if we happened to miss one, it can be added through the Registry."
A response to my request for the key came from Jimmy Bergmark;
Edit, remove or add the scales you want in the registry at this location:
NOTE: As always, be very careful when editing registry. Incorrect editing may severe damage to the operating system or software operation. Backup before making any changes. See Regedit Help for "Best practices when editing the Registry".