Percent complete IF Formula



Please assist with a IF formula for a total percentage completion of Task1, Task, Task3.

If Task1 is 330 Days it is considered 100% complete. IF Task1 is 331 days than the task is 0% complete. I would then take Task1-Task3 completion % and average to show total percentage. Limiting this to just few helper columns would be helpful as there a multiple tasks which are being tracked. Thanks for your help.