< 40% is not "too far", it's just outside the optimal standing charge range for battery longevity, which studies have found is 40-80%.
In other words you can use your battery from 100% all the way down to shutdown- but the more time it spends in the 40-80% range, the longer its service life.
I don't know how much reserve they keep on hand, but what your OS reports as 0% charge is almost never actually that low.