Allow a different post-retirement annual expense
Since many of us plan on retiring abroad thus greatly reducing our annual expenses (and even if we don't move, many of us do reduce expenses a lot), the calculator should not assume by default that the current expense rate is also the post-retirement expense rate.
I had the same issue with this. I'm living frugal right now but don't plan to be as frugal as I am today when I retire because I see little point in that. Things like regular travelling will surely increase my expenses. I don't see a way where this can be factored into my FIRE calculations.
Under the chart breakdown I can see one of the assumptions is:
"Your current annual expenses equal your annual expenses in retirement"
This is the issue, because it isn't suitable for me.
David Johnson commented
I had the same problem so I made my own calculator. I'm not trying to steal any views or anything but if it's helpful, I put it online at http://RetireTimeCalc.com