You are correct, manual states 1/4" to 9/16" below fill hole. I like the hex key idea. I wonder how things would be affected by just filling until it comes out.
Originally Posted by Astro14
On my 2000 Expedition, the proper axle fill was something like 1/4" to 1/2" below the fill plug level. So, it wasn't just a "fill it until it runs out" kinda' deal. I actually used a very small hex key wrench to check the level after adding the specified quantity. Check your factory service manual, or subscribe to an on-line servicing information site, to get the correct level for your Ford.