In an alambic can go up to 30HL of wine, but only a maximum of 25HL is allowed. After the first distillation 7HL brouillis is obtained out of this 25HL of wine (2/7th part). The same applies to the second distillation: out of 25HL of brouillis you gain 7HL eau-de-vie. So that is two times 2/7th. When you begin with 100 liters of wine and multiply two times with 2/7th the result is 8.16 liter. But in reality the yield is somewhat higher than that.
As can be seen in the above diagram, têtes and queues are not thrown away, but are used again in the next distillation-batch. That is why not just 8.16 liters are obtained, but somewhat more: 10 liters. So to fill one 0.7L bottle of cognac, you need 7 liters of wine.That is, when it is a very young cognac. If the cognac ages for a long time, there is an addional loss of 2-4% each year through evaporation: the angel’s share!