Nintex noob here. Just spent a bit of time figuring this out but I am curious as to why one formula works and one does not:
This works:
fn-If(fn-Or(fn-IsNullOrEmpty(Stage),fn-Equals(Stage,'1')),false,true)
This doesn't work (it always comes back false but should be true if stage is anything but null or 1):
fn-Not(fn-Or(fn-IsNullOrEmpty(Stage),fn-Equals(Stage,'1')))