While it's hard to predict, the state of the economy when you retire can affect your pension's buying power. Inflation, interest rates, and market conditions all play a part. If you're retiring in a period of economic downturn, it may be wise to be more conservative with your pension withdrawals, at least initially. You should also regularly adjust your cashflow model with help from your adviser.
The ‘4% rule’ suggests that if you withdraw 4% of your pension pot in the first year and adjust this amount for inflation annually, your savings are expected to last for 30 years. However, this assumes your spending is linear, whereas the reality is your spending is likely to alter as you age. Typically, your expenditure is high in early retirement, then reduces as you age, then potentially increases if a nursing home is required for later life. A financial adviser can help you ensure your health, outgoings, attitude to investment risk, and ability to withstand financial losses are built into these assumptions as you progress through retirement.