Lead Time Formula vs Projected Lead Time

If you specify both a Lead Time formula & Projected lead time in the Maintain Inventory, which does MRP consider? I did a average lead time calculation based on the last 15 months using the PO date vs the Receipt date and was going to update the Projected Lead Times that are entered into the system. But now I am considering using the LT Formula. If I do, should I remove the Projected LT? What happens if I leave it?

I know I can test this on our test db, but I wanted to hear from the experienced users. Anyone have some input on this?