20 years is the shortest duration from PEA to depletion. As to what criteria cpf uses to determine to give u the shortest 20years, or add some more years to say 23, 25, 28, 30.... I don't know. Whatever cpf dishes out, no harm to ask for shorter duration if that is what u wanted and await their reply.
in CPF's reply to my query, they alluded to the additional interest having effect of extending payout duration :
"My Qn: Is it correct that the maximum monthly payout is computed to last 20 years from my PEA ie til 83yo ?
CPF Ans: Yes, that's right. CPF members can apply to increase their CPF monthly income provided their income can last at least 20 years from their payout eligibility age, or at least another 5 years from the time of application, whichever ends later; subject to terms and conditions.
Notwithstanding, the resulting payout duration may vary from 20 years due to new policies introduced, which is shown below for your quick reference:
• Extra Interest (EI): An extra 1% per annum on the first $60,000 of a member’s combined balances [with up to $20,000 from Ordinary Account(OA)]
• Additional Extra Interest (AEI): CPF members aged 55 and above will also earn an additional 1% extra interest on the first $30,000 of their combined balances (with up to $20,000 from the OA)
The purpose of EI and AEI is to extend the duration of members’ payouts, to ensure that they do not outlive their retirement savings as much as possible.
Hence, based on the scenario above, the payout duration will last till age 84."