Planescape: Across the Multiverse
Toughness is ranked by two numbers: the first represents the non-lethal toughness of a character, the second the lethal. The first is calculated normally. The second by dividing the first and rounding up.
Example: Symeon has a Vigor of d8. This makes his basic toughness 6. Versus lethal damage his toughness would be half of this, or 3.
Magic damage is calculated against the better of either raw toughness or lethal toughness plus armor.
Adopted from Craig_the_lucky’s House Rules