The adjustments in OD&D (except for Charisma) could pretty closely be modeled with 1d6: 1 = 3-6 (major penalty: -20% XP, -1 hp/die), 2 = 7-8 (minor penalty: -10% XP, -1 missiles), 3-4 = 9-12 (no adjustment), 5 = 13-14 (minor bonus: +5% XP, will withstand adversity, +1 missiles), 6 = 15-18 (major...