Not being facetious, but what's the point?!
In any case, unless you know what your reckonable salary is going to be throughout the whole of your service, and at the date you retire, you won't be able to calculate it.
Under a final salary defined benefit scheme - such as most PS workers at present have, you get a pension entitlement of 1/80 for each year of service, up to a max of 40/80 if you serve 40+ years. And then when you retire your salary on pension will be 40/80 x Final salary. So if you get promoted in your last couple of years of service, you could get a much higher pension than you would have if you weren't promoted.
Whereas under a career average DB scheme, you still get 1/80 for each year etc... but your salary on pension will be 40/80 (assuming you work the full 40 years) x Average Salary. In this case being promoted for the last couple of years will earn you only a marginally higher pension...
Consider an example of someone who starts as an Executive Officer in the Civil Service, works for 40 years, and gets promoted every ten years:
Under a final salary DB scheme they will retire at point LSI1 of the Principal Officer scale at €100,446. So their pension will be based on 40/80 of €100,446 = €50,223.
The same person on a career average DB scheme will retire on the same final salary, but their pension will be (1/80 x 30,516 + 1/80 x 32,687.... etc), and a quick guesstimate is that their pension will be €32,292.
So in that specific instance the career average DB pension will be 36% lower than a final salary DB pension. But at the end of the day it's all a redundant exercise, the day of the final salary DB pension are gone.