I have used that retirement planner (and a 1000 others...), and it seems to work OK. I believe it assume a constant rate of return.
Never rely on past performance, Monte Carlo, or steady return calculators alone. Use them all, and make sure you are ready to go no matter what happens. Remember, the future is unknown.
And make sure you have a bit of cushion. The amount of cushion I have would probably make others think it's too much, but I know I will not be coming back to cubicle work. Ever.
FIRE no later than 7/5/2016
at 56 (done), securing '16 401K match (done), getting '15 401K match (done), LTI Bonus (done), Perf bonus (done), maxing out 401K (done), picking up 1,000 hours to get another year of pension (done), July 1st benefits (vacation day, healthcare) (done), July 4th holiday. 0 days left. (done) OFFICIALLY RETIRED 7/5/2016!!