Section 39(1)(b) in The Navy (Pension) Regulations, 1964
(b)if the qualifying service is less than twenty years, the service element shall be assessed by making deductions from the service pension which would have been admissible to him on his retirement with twenty years qualifying service, such deductions being made in the manner, and at double the rate, specified in regulation 23 for each year or part of a year by which the qualifying service falls short of twenty years.