This free Enough Simulation Tool can help you get a feel for how much is enough for you, based on your own needs. Note that the link above will ask you to make your own copy, so the spreadsheet will become yours and is totally private.
A spreadsheet can never replace sophisticated financial planning, as it can't capture all the complex nuances of different return rates, taxes, etc (without getting really complex). However, the goal of this spreadsheet is to provide users with a simple way to get a first rough approximation, which can then be used as a basis for going into more detail.
This Enough Simulation example can serve as a guide on how to fill in the Enough Simulation tool.
The example case also shows what happens in these scenarios:
'Continue as is' - just personal / family consumption
'Be generous' - giving a substantial amount each year
'De-accumulation' - gradually reducing wealth over a number of years
For more information contact Robert Boogaard (robert@enough-project.org)