I'm assuming the tapes are rated at 12./2.4? The 2.4 relies on getting 2:1 compression, something that rarely happens in the real world. See http://seer.support.veritas.com/docs/199542.htm
If you are getting about 4.9GB on three tapes, you are achieving about 1.36:1 I generally assume about 1.3:1 until proven otherwise, so you are right in the ball park. Add another tape, and you should get a complete backup