As mentioned, its in the AFM.
Usually (my experience ) in a graph form allowing for differing conditions. I end up memorizing the worst case scenario altitude as my bottom number. Most jet's I'm aware of can maintain suffecient altitude to clear the highest obstacle in the continental US. That said an engine failure at altitude isn't the biggest concern. Its losing one shortly after T/O from a mountainous airport. There companies use a few different methods for driftdown. If you lose an engine from A to B, divurt to ABC, from C to D go to DEF.