If you want to do "days endowed," in contrast, do this easy (though dirty) calculation:
How much do you want to spend per year? For instance, "$40,000."
Divide $40,000 by 365, which is how much you want to spend per day: $110
If you are going to use the 4% rule, then you need 25X this amount to "endow" that day (because 4%X25=100%, but don't worry if you don't understand that--just trust me on the 25): $110 X 25 = $2750.
So every $2750 you save is one day you have endowed.
How many days have you endowed? Take your current nest egg, and divide by $2750. Imagine it is $500,000: $500,000/$2750 = 181 days so far, and 365-181= 184 to go.
(By the way, it is "dirty" because you're not accounting for inflation. If you are far away from retiring, that $40,000 amount is going to go up. No worries--just every few years, redo the calculations with an updated yearly spend.)