[BrianG]

The last gear does take up some room, so 80% sounds about right. I usually fill my diffs until the fluid just covers the spider gear shafts. I also spin the gear a little to remove any air pockets, which would make the fluid level fall a little.

Many diffs have a set screw at one end (for the gear pin) - maybe you could use that as a fill point? Wouldn't have to worry about the 80% thing that way...