Lynx gives you many different methods to use to determine the next delivery date for any given tank. Each tank can use whichever routing method works best and you may well have different combinations of routing methods within a single route.
These should be reviewed periodically to insure accuracy. A good way to do this is to run report DR02 "Delivery History" and check the Efficiency Rating. If they are consistently low (like under 50%), you might want to change or modify the Routing Method used for that particular tank.
For a further explanation of a routing method, click its link below: