I'm a big fan of both EA and FIRE, so this is awesome!
One way the tool could be improved is by allowing to adjust for-tax deductibility of donations.
I'm not quite sure how the calculator would be adjusted, but the relevant metrics seem to be.
The distinction between if it applies to net or gross income might be necessary, because countries differ to the specification of tax-deductibility of donations.
Maybe most relevant excerpt for the forum (taken from 01:08:45, might contain some mistakes):