full_time_equivalent = models.DecimalField(
    max_digits=5,
    decimal_places=2,
    default=100,
    validators=[
        MinValueValidator(Decimal(0)),
        MaxValueValidator(Decimal(100))
    ]
)

def save(self, *args, **kwargs):
    # Run validations
    self.full_clean()
    return super().save(*args, **kwargs)

project2_membership = ProjectMembership(
        user=self.new_user,
        project=project2,
        is_project_manager=False,
        full_time_equivalent=10.01
    )

Decimal('10.0099999999999997868371792719699442386627197265625')


django.core.exceptions.ValidationError:
{'full_time_equivalent': ['Ensure that there are no more than 5 digits in total.']