Banky team Reviewed editorial answerThe amount needed depends on the rate that applies to the term and payout you choose: the term and the payout frequency, and sometimes the amount itself, because larger amounts can fall in a band with a different rate.
Hypothetical example with monthly payout at 20% a year: every 100 pounds pays about 1.67 pounds a month, so each pound of monthly income needs about 60 pounds placed. At 18% that becomes about 66.7 pounds. A small difference in the rate visibly changes the amount needed.
Other things to account for: with a stepped certificate the payout falls each year, so an amount that is enough in year 1 may not be enough in the last year. With a variable rate the payout can change with the reference rate. If the certificate is sold in multiples, the amount is rounded to the multiples. And this money will be locked for the term and breaking early has a cost, so consider whether some of it should stay available.
The practical way: set the term, ask the bank for the monthly payout on a given amount, adjust the amount until you reach the income you need, and check the rate schedule, the minimum and its multiples in the key-facts statement.