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.
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