Another way to look at it is that the level should be at or above
the lower line when cold (stone cold), and at or below
the upper line when hot (at full operating temperature).
As ChrisC indicated, if you think you're under (or over) filled, you can rock the bike side to side on the centerstand to see just how under or over you are.
A few degrees of angle should
bring the level into view, as it takes only a few ounces
of oil, one way or the other, to see an empty sight glass... or a window full of the stuff.