There is a black rod holder (very cheap), that you can get at most survey suppliers, that attaches to the plate on your tripod. You attach the rod to the device and plumb center the frog-eye bubble. Rotate the rod 180° and check it. If the bubble is out, adjust it only ½ way to the correct position. Repeat the procedure until the bubble remains centered at all rotation points.
Note: When you adjust the bubble, adjust it only ½ way. If you adjust it to the center you have just mirrored your error when you rotate it 180°, again.