If you double click the User Defined Variable in Project
Manager, you can change the default there. I think what you want
though is a different value for different builds. In the first page
of the wizard when you generate there is a Variables button, you
can change it there for for that build.
You could create different layouts for different builds and
each will retain its own value. Craig Clarke has written an article
on this which will shortly be published on Adobe's site. Hope this
helps until then as I know Craig, who has more knowledge of this
area, is currently on holiday.
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