the level has to be within sight of the bottom of the fill plug hole
as long as you can see it then the level should be ok, i would not fill it all the way though, oil expands when heated up and level will rise and if you fill it too high then it may blow out of the case breather.

using a bright led flashlight and maybe a measuring stick or something to stick in there to give you and idea of where the oil is helps alot ;o)

while adding oil (if you have to) pour some in, let it settle before checking it, repeat if needed till level is where you want it.

yes i don't know why they do not provide a sight glass for these, it surely is needed. :o)