Its a Formula!
This is the appropriate way to calculate how many calories/day you have to consume.
The average loss per week is around 2 pounds/week so you would have 140 days of reduced calories intake (20 weeks x 7 days).
1) subtract goal weight from current weight
2) multiply 40 x 3500=140, 000
3) 140, 000 by 140 equals 1000.
For you, it would be 1000 cal/day, which is not healthy. Average is 1400/cal a day.