Overfilling a differential?

I was skimming through kyosho's instruction manual for the mp777 sp2 and it stated "fill up to 80%". It diagram does not show the last gear in there, so maybe they just don't want you wasting fluid?
Essentially, I am not used to fluid filled diffs (my other RC is a t4), so I wanted to make sure if I rebuilt the diff properly.... Is it suposed to be full? or not? or does it not matter?

Thanks