Hi Sidzer,
If you have 12/13 years teaching it seems to be that you should be paying A1 PRSI. (You will be able to see from your payslip, it will either be A1 or D1). If it is D1 the figures you have done out are more or less correct, however if you are A1, your pension is co-ordinated with State benefits and therefore it is a different calculation. Based on the information you have given I would estimate a pension of around 4800 p/annum. The reason for this is that you will be entitled to a State Contributary Pension at 68 in addition to your Superannuation Pension.
Remember you will also have the option of deferring your superannuation benefits until your normal retirement age instead of taking the cost neutral pension if you decide to retire early.
You should be able to get figures from Dept Education or VEC (whoever your employer is). You should maybe also get impartial financial advice as to whether you should supplement through NSP/AVC/PRSA. Make sure your adviser understands implications of cost neutral retirement, Revenue rules etc; as goalposts seem to be changing quite a bit over the past few years!
Good luck with your decision, hope this helps.