class Tarifa(models.Model):
    limite_i = models.DecimalField(max_digits=10, decimal_places=2)
    limite_s = models.DecimalField(max_digits=10, decimal_places=2)


class Calculadora(models.Model):
    tarifa = models.ForeignKey(Tarifa, on_delete=models.CASCADE)
    base = models.DecimalField(max_digits=10, decimal_places=2, null=True, blank=True)
    limite = models.DecimalField(max_digits=10, decimal_places=2, null=True, blank=True)

qsl1 = Calculadora.objects.values_list('base')

<QuerySet [(Decimal('3000.00'),), (Decimal('5000.00'),), (Decimal('1000.00'),)]>

qsl2 = Tarifa.objects.filter(Q(limite_s__gte=qsl1) & Q(limite_i_isr__lte=qsl1)).values_list('limite_i')