First of all, it doesn't matter that how much have you scored in your 12th standard. The National Defence Academy doesnt prescribe a minimum percentage criteria to appear in the exam. What matters the most is your will to join the defence forces. The NDA/ NA exam is conducted in every 6 months by the UPSC. If you clear that you get a call for SSB, and after then Medical examination subsequently. The merit list being the final hurdle before you join the army.
I'd suggest you to stop worrying about your percentage in 12th and prepare for your written exam. The notification for NDA 2 2017 will be out on 30.6.2017.