Are you checking the level before or after it is at operating temp? You might also check the vacuum line to the modulator. The rubber hose might have become deteriorated or contaminated, causing a leak.
I checked it when the car was warm but it is hard to read. Seemed to be above the full line. I wanted to add some transx to help smooth out the shifting temporarily but don't want to over fill it and blow a seal.
my money's on the rubber hose to the modulator. Like Buebird said, check it. Maybe it just popped off, or maybe it needs to be replaced. More often than not, that's the cause of the problem you describe.