What lag should be used in determining the forecast accuracy and what lag does the software use in calculating the accuracy?

The answer to this is with the planning process and not the software. The customer of the forecast determines the lag period for accuracy. For instance the factory or plant may need the accurate forecast with respect to the production or RM procurement lead times and may extend upto 3-6 months. However, logistics team may need it on weekly or monthly basis for dispatches and deployment. However, a good forecasting software like Planamind does, should be able to show you the forecast accuracy for different lag periods on the report, using analytics platform.