We have MS Level 1 parts that are manufactured and many that are purchased (finished goods).
All have a demand time of 2 days.
MS Level 1 parts that are manufactured have a planning time fence of 20 days, and the planning time fence of those that are purchased reflect the purchase lead times of the parts, typically 50+ days, many above 100 days.
For manufactured parts we tend to set a production plan for the next 3 weeks, and will not change the forecast in the planning time fence.
For purchased parts with such a long lead time (planning time fence) we believe that we should make changes to the forecast if we expect that demand will change. By making such changes and being able to see the forecast in the planning time fence we would be able to communicate risks, such as when we see we will go into negative on-hand, and we may be able to act to expedite purchase orders, for example choosing a faster mode of transport.
What is the best practice for managing the planning time fence and forecasts within the planning time fence for purchased parts with such long lead times?