Just want to point to a nice post by fellow MVP Dr Scott Hamilton on the MSDynamicsWorld.com website: Master Scheduling and the Due Dates for Safety Stock Requirements in Microsoft Dynamics AX & 365 for Finance and Operations, Enterprise. It’s a really nice summary of the options for setting up safety stock and highlights the all-important ‘Fulfil minimum’ parameter. He also shows a neat little work-around using the Minimum key to set (or offset) the due date of the safety stock.
My own attempt at describing the ‘Fulfil minimum’ options are here: https://timsaxblog.wordpress.com/2016/02/19/item-safety-stock-in-ax-2012/.
Finally, if you are working in recent D365FOE environments you may want to check out the Safety stock replenishment enhancements in 7.3, see (https://docs.microsoft.com/en-us/dynamics365/unified-operations/fin-and-ops/get-started/whats-new-application-7.3-update).