This is one of the ways to work out the figure to provide for the reasonable requirements of the spouse in need of ancillary relief.
An actuarial expert is engaged to work out a lump sum which would, on investing the lump sum, produce an income enough to meet the spouse’s needs for life. The calculation has to consider factors such as life expectancy, rates of inflation, return on investments, growth of capital and incidence of income tax. (Duxbury v Duxbury  2 All ER 77)
The Duxbury calculation provided a starting point upon which the court could assess the ‘reasonable requirements’ of a wife.
Useful Update for Hong Kong
See Felix Chan & Wai-sum Chan Using Actuarial Tables in Matrimonial Finance Disputes
See Hong Kong Lawyer – Sweet & Maxwell
A new (January 2019) and very useful site in HK Re Duxbury – see Azan Marwah et al Duxbury etc