I love this idea.
For those of you calculating the cost of your "forever days" -- do you use your current (actual) spending, or your planned retirement spending?
I'm far enough away from FI that our retirement budget is guesstimate, but I'm into this idea and would like to calculate our "forever days." I do expect some of our costs to come down in FI. For one, we will probably take a break from contributing to charity, or scale it back dramatically. (However, healthcare costs will increase.)
Our FI plans are difficult to put a real number on, in general, as I expect we will phase out of the workforce via part time work -- and we are promised fairly substantial pensions from our jobs even with early retirement, but when calculating our FI number and "forever days" I tend to ignore those entirely.
I suppose this is the sort of thing where even as a guesstimate its a pretty effective motivator --- I love the idea of combining it with the wall chart! I may be breaking out some markers tonight...