[Carl n Susan]

My Ford tells you the level is OK, below 1/2, or you have 500 miles to go. You do not want to run out of DEF. I check the level each day when driving. When it says it is below 1/2 full, I wait 100 miles (1/2 empty is not really 1/2 empty) and then top it off. This has worked great for 100K+ miles.

You can buy a DEF fill nozzle from Ford, but much cheaper on Amazon, that doesn't spill DEF if the tank fills up before all of the 2 1/2 jug is gone.