You can try it like this:
```python
PERCENT_BASE_CHOICES = (
    ('WITHOUT_INVALID', ugettext_lazy('...'),
    ('WITH_INVALID', ugettext_lazy('...'),
    ('DISABLED', ugettext_lazy('...'))

class CollectDefaultVotes(models.Model):
    ...
    def get_percent_base_choice(self):
        """
        Returns one of the three strings in PERCENT_BASE_CHOICES.
        """
        raise NotImplementedError('You have to provide a get_percent_base_choice() method')

    def print_votesvalid(self):
        if self.get_percent_base_choice() == 'WITHOUT_INVALID':
            value = ...
        elif self.get_percent_base_choice() == 'WITH_INVALID':
            value = ...
        else:
            value = ...
        return value
```

Then you have to override the get_percent_base_choice in MotionPoll and AssignmentPoll and there use the config value. In the config definitions you can use the CHOICES tuple here to use less code.