Except in the Help Station on the right-hand side it states:
Final Pensionable Salary
Your final pensionable earnings are the greatest amount of pensionable earnings that has been earned in 365 consecutive days falling within the last three years. This calculator does not take this into account and assumes your final pensionable pay is the pay level you have input. See JSP 764, Part 1, para 0305 for more detail.