My first post and it is nothing to do with aviation! But this subject did grab my attention over the past couple of years as a recent APFS05 retiree aged 54.
As a previous poster said, it is the most remunerative consecutive 365 days over the previous 3 years that count in determining your pension under 05. Additionally the figures get adjusted for inflation. So this might become relevant for some people under a pay freeze but with inflation marching on. It is in JSP764 Part 1 para 0305, available on the internet or intranet.
I do suggest you get your mind around the JSP if only so you know whether the lump sum and pension/EDP you get are roughly right, as for me Glasgow just gave me the final numbers without explaining their calculation.
Also if you are doing comparisons of retirement dates vs benefits then get your mind around discounted cash flow if you do not already know what it means. Money now is more valuable than money later. But of course whether you need money now is highly relevant.
And finally be aware that the change from rpi to cpi, assuming it happens, does affect the sums. For me a change to cpi will mean the the second lump sum I get in 10 years time will be worth about 10% less in real terms than if rpi applies.