Depends on your party though. It is possible your players won't use consumable items of abilities (ie spell slots) for fear of "wasting" them. This lowers their effectiveness dramatically and throws formulas off. Or they panic and use all of their items every fight and are out of resources constantly. The DMG was written with assumptions that don't always hold true.
HP is a resource too though.
If a party uses no spells and takes double the normal amount of damage (or vice versa) they have still expended the same overall percentage of their party's resources.