Formula for Calculating the Reorder Level
The reorder level is defined as the sum of the safety stock plus the requirement forecast within the replenishment lead time (see formula (17)).
A forecast was carried out on a monthly basis. A month has 30 days in the case of external procurement.
Safety stock 
100 

Forecast 
1st subsequent period 
200 
2nd subsequent periode 
00 

3rd subsequent period 
400 

Replenishment lead time 
40 days 
0/30 
30/30 + 10/30 
(an entire monthly requirement + a part of the the following month)
Reorder level = 100 + 30/30 * 200 + 10/30 * 300 = 400