Solution : https://service.sap.com/sap/support/notes/502897 (SAP Service marketplace login required)
Key words : 
adjustment billing document, attached advance correction, user parameter mm_fcst_round, terms material forecast, material forecast, forecast values, transactions mp30, low consumption, reorder point, safety stock
Related Notes : 
       
| 813294 | Material forecast: Rounding of the forecast values | 
| 529729 | MD62 forecast value in demand management rounded up | 
| 528153 | Material forecast: Storing the rounded values | 
| 521579 | MD70 Forecast value rounded up in demand management | 
| 428995 | Correction to Note 383823 | 
| 393320 | Advice: Threshold value for rounding in material forecast | 
| 383823 | MP38: Restriction according to materials | 
| 93765 | Forecast values: rounding decimal places |