Normally the transmitter oil level will drop. Not that much in 800 miles though.
Normally some of the oil goes into the u-joint clamshell and mixes with the grease lubing the joint. But, a sealed bearing should stop that.
So, I too would recommend checking the vent in the shift tower. If the clamshell isn't leaking or throwing grease/oil, then the oil is probably going into the driveshaft tube or bellhousing.
But, as far as the level is concerned, anywhere between A and B. Once it reaches B, bring it back up to A.