The IMG can be set in SM30 - V_438M_S

You define per plant and per MRP group the percentage of the safety stock that is to be available for planning.  This helps to avoid a situation where order proposals are created unnecessarily by the system to cover small shortage quantities.

For example:

                                      Mat. 1         Mat. 2         Mat. 3 

Safety stock:                  100 pc         100 pc        100 pc 
 

available for planning:       0%              50%           50% 
 

Shortage quantity             1 pc             1 pc           51 pc 

Order proposal quantity 
for lot-for-lot order qty    1 pc             0 pc           51 pc 
 

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.sap-img.com