What Is the Best Approach for Estimating Retirement Expenses

What is the best approach for estimating retirement expenses?

A) Seventy-Eighty percent of salary
B) Seventy-Eighty percent of pre-retirement expenses
C) Divide fixed and flexible expenses into "key" separate categories and estimate expenses for each category
D) One hundred percent of pre-retirement expenses
E) One hundred and fifty of pre-retirement expenses