I used 70% as an example. If that makes your output too large to fit (when you add in the extras) you have to make it less... Generally that's okay -- as movies that take a lot of space (but aren't long) can usually be shrunk by more without losing too much quality.
By the way -- this same problem exists in the other packages and will assuredly exist when
DVD2one adds that feature. You can't argue with physics...