Your avoidance is more than fine for Naxx10. For some of the later fights (Saph, Kel'Thuzad, Malygos) Avoidance does little to nothing anyway, and Stam stacking is prefered. There's only 1 fight I can think of where it's recommended to stack Avoidance (Patchwerk - 25), all other fights really do not need it.
As for threat: Your HP is quite high for tanking 10 mans. If you do not plan on tanking 25 mans in the near future, you should consider replacing some Stamina with Expertise/Hit (red and yellow slots). Accuracy is a great enchant for your weapon if you can afford it.
Failure is not an option. It comes bundled with the software.