Yeah, that's a vent. Standard rule of thumb is to add four quarts. Then start the engine and with it idling in Park add three or four more. If the transmission has just been rebuilt, get in the car and put the shifter iin Park, N, R, D, 2,1 then back in Park. You don't have to go anywhere, just put it in all the gears. Not necessary to do this is you're just putting it back in or installing a used unit but not a bad idea.
Now with the engine running in Park, check the level on the stick (and check for leaks underneath) and fill a quart at a time until it's at least up to the lower mark on the stick.
Then test drive two or three miles, recheck the level and add if needed. Recheck for leaks too.
As I have laid it out is exactly what transmission shops do.
The reason yours overflowed with five quarts is that a LOT of the total fluid stays in the torque converter. The pan never really has more than four quarts in it at any time. Unlike a car engine where you can count on most of the oil sitting in the pan. | 
