The method provides a planning system where production plans and safety stocks are determined simultaneously, thus resulting in coupled target stock calculation and production plan. The closer the manufacturing process gets to the next production event, the lower the safety stock needs to be. In this approach, assumptions on the average production lead time—which might be wrong—do not need to be made. Indeed, if the production plan and the safety stock are simultaneously computed, it is clear when the next production is going to occur. As a result, production variability only needs to be considered in relation to the probability of unexpected events in manufacturing (e.g. a machine breakdown) as opposed to taking provision for variability of the production cycle due to situations that can be anticipated at the planning step.