What you can do instead of fiddling with the Registry, would be providing a Print me button on the forms, and use printParams() to specify the scaling. Then it would be a question of instructions to make the users use that button.
If modifying the Registry seems easier, I know that there is a document describing the Registry settings, but I have been apparently too dumb to find it the other day… Maybe someone would jump in here.
Hope this can help.
Max Wyss.