Following is the logic for continuing income sources .
We take into account the income of the spouse and use the following formula
PV = PMT * ( ( 1 - ( ( 1 + r ) ^ - n ) ) / r )
r = Inflation adjusted Return (Inflation Adjusted Return = ((1+Expected Return On Investment)/(1+Expected Inflation))-1)
n = Group Head's Retirement Age - Current Age
PMT= Total Annual Income of the Dependant
There is an "Income Adjustment" factor that allows you to mention a %. E.g. if you say 80% then the amount will be "Total Annual Income" * 80%. This gives you control over the figures.