Problem is the known knowns, providing for the unknown knowns not to mention unknown unknowns.
Taken in turn, while I agree with the subtraction of debt, commute costs to work, pension contributions and savings, people tend to plan a lot of travel given their excess free time and therefore initial costs and that type of lifestyle can be expensive. Then you have further down the retirement road the provision for medical care and / nursing home. Thereafter, it would be the unknown situations that arise, funding a son/daughters mortgage or marriage breakdown (I known one of relatives has this situation).
My own view for what its worth would be to actively fund for the "active retirement years" Age 55 to 75, then divert funding to increased medical care in the age 75 to 85, thereafter once your home covers nursing home costs you have provided accordingly for yourself. This would cover most situations bar unknown unknowns.
In Ireland we always seem to provide inheritance, however my plan will not. My view on this is to avail of the annual gift exemption and ensure family is assisted when they need cash most rather than "save for the rainy day" . The other part of my plan is to find a country that is not so WCE (wet, cold and expensive)