Originally Posted by Medic97
One more thing to add: According to Ford service; in order for the gauge to reset when adding DEF, one must fill tank. Otherwise if just adding fluid, the system will not know how much it has.
That is actually incorrect. There have been some owners that have reported they have filled up mid way and it never reset as well as some who have hit the 500 mileage warning and it didn't reset for several miles.
However, on my 2015, I wait till I get to about 200-225 miles left and on my 5.2 gallon tank, it takes roughly 4.75 gallons. When I pour it in, I leave my dash on and I see it jump right up and then go to "ok". There is no waiting and you don't have to fill it up to it to register that it is increasing in volume.