vand wrote:To know how much I need today to pay that off, I simply apply a suitable discount rate to the future capital and interest payments to arrive at the net present value I need today.
SWR is much the same. The easiest way to measure it is to use compounded inflation adjusted (real) values for gains/losses and deduct 4%/year from that. 1.0 with 0% annualised real gain with 0.04 deducted each year ... lasts 25 years.
1.0
0.96
0.92
... progression.
The 'official' SWR if there is one as such is based on a assumed 60/40 stock/bond asset allocation and a 30 year time horizon, where the worst historic cases drew down to zero remaining. That could be considered as too long a horizon, most 65 year old retirees wont live to being 90 (25 years) let alone 95. Or if they do then often their home value might cover 'all-inclusive' care-home costs such that other spending is zero anyway.
0% real for 25 years covering 4% SWR may seem relatively conservative/safe, however what those that suggest it will be lower in forward time are saying is that a 'bad sequence of returns' is anticipated. If for instance you actually endured a -50% real loss in the earlier years that's similar to starting with 0.5 instead of 1.0 and to compensate for that the SWR has to be reduced to 2% instead ... or something along those lines. That however is very pessimistic, large earlier years real declines followed by 0% real subsequent returns for (2.5+) decades. Whilst one or the other may reasonably occur, its less likely that both will occur. Large declines are more inclined to see reasonable/good subsequent gains, as might large gains just prior to retiring be inclined to subsequently seeing below average real gains.
Your approach can be reversed, assume that a future spending can be adjusted by the future expected real interest rate back down to a present day value. Assuming 0% real makes that easy, if you require 25 years of inflation adjusted £20K/year then it costs £500K of present day money to cover that. In some later years that might be further discounted, such as a £10K state pension being available after 5 years perhaps, so £20K/year for 5 years and £10K/year thereafter for 20 years, present day cost of £300K. For a 60 year old that is due to receive a £10K occupational pension at age 65, and another £10K/year state pension from age 67, who has £20K/year spending, they only need £140K of present day money in order to retire. Or a 60 year old with a immediate £10K/year occupational pension along with a £10K/year state pension from age 67, only needs £60K.
The errors as I see it are that instead of promoting security/safety, consecutive governments have induced greater risks/uncertainties, elected by the many to serve the few (businesses that 'bribe' MP's). Inflation linked occupational pension schemes killed off leaves more profits for companies to distribute to shareholders or to further line the pockets of those with more than enough wealth already, at the expense of greater risk upon individuals. Along with increases in state pension age and/or state pension reductions. By not uplifting the state pension by inflation both existing and all future individuals lose out - individually have to have more in order to plug that hole. The American way, where individuals risks are increased for the benefit of the few at the cost to many. Accordingly Capitalism and the likes of Fiat currencies are being pushed towards extinction with the rise of the many opting for 'change'.
I see a rising probability of a more extremist Labour government being voted into government, one where the likes of private ownership of homes will be replaced by broader collective insurance against health, housing and spending. A significant and relatively rapid swing away from the Capitalist/Fiat pendulum swing. I was relieved that didn't occur at the last General Election, but as the next approaches so that fear is being rekindled. I don't consider myself to be 'rich', rather 'comfortable', but above 'average' i.e. we own our own home and have some savings/investments. The rich will simply flight the country under such government to leave the 'average' as being the rich to pay the bill. Therein lays the 4% SWR risk IMO.