[Jafir]

Classic/Kunark/Velious Agility gave both Avoidance and Mitigation AC. Whether or not the formula on P99 adheres to that, I do not know. Here is a reference to mid-Velious, which still had a hard cap on Mitigation AC:

defense_agility_bonus method

if GetAGI() < 40: # penalty
  return (25 * (GetAGI() - 40)) / 40

if GetAGI() <= 60:
  return 0

if GetAGI() <= 74:
  bonus = 28 - (200 - GetAGI()) / 5
  return 2 * bonus / 3

# so 75+
bonus = max(200 - GetAGI(), 0)
if GetLevel() >= 40:
  bonus = 80 - bonus / 5
elif GetLevel() >= 20:
  bonus = 70 - bonus / 5
elif GetLevel() >= 7:
  bonus = 55 - bonus / 5
else:
  bonus = 35 - bonus / 5
return 2 * bonus / 3

compute_defense method

skill = max(GetSkill(SkillDefense), 0)
bonus = 400 * skill / 225 + defense_agility_bonus()

if IsClient():
  drunk = float(GetIntoxication() / 2)
  if drunk > 20.0:
    redux = min((110.0 - drunk) * 0.01, 1.0)
    bonus = int(bonus * redux)

return max(bonus, 1)

ac method

item_ac = 0
for slot in range(0, 21):
  item = GetItemPossession(slot)
  if item.IsValid and item.GetItemClass() == ItemClassCommon:
    item_ac = item_ac + item.GetItemAC()

if not IsMage():
  bonus = 4 * item_ac / 3

if IsClient() and bonus > 6 * GetLevel() + 25:
  bonus = 6 * GetLevel() + 25

weight_hardcap = 30
weight_softcap = 14

if GetLevel() > 59:
  weight_hardcap = 45
  weight_softcap = 24
elif GetLevel() > 54:
  weight_hardcap = 40
  weight_softcap = 20
elif GetLevel() > 50:
  weight_hardcap = 38
  weight_softcap = 18
elif GetLevel() > 44:
  weight_hardcap = 36
  weight_softcap = 17
elif GetLevel() > 29:
  weight_hardcap = 34
  weight_softcap = 16
elif GetLevel() > 14:
  weight_hardcap = 32
  weight_softcap = 15

if GetClass() == MONK:
  weight = GetWeight()
  if weight < weight_hardcap - 1:
    weight_bonus = float(level + 5)
    if weight > weight_softcap:
      temp_mod = min(float(weight - weight_softcap) * 6.66667, 100.0)
      weight_bonus = max(0.0, weight_bonus * ((100.0 - temp_mod) * 0.01))
    bonus = bonus + int(weight_bonus * 11.0 * 0.1)

if bonus < 0:
  bonus = 0

if GetClass() == MONK:
  weight = GetWeight()
  if weight > weight_hardcap + 1:
    weight_penalty = level + 5
    temp_mod = min(1.0, (float(weight) - 20.0) * 0.01)
    bonus = bonus - int(temp_mod)

if GetClass() == ROGUE:
  agi = GetAGI()
  if agi > 75 and GetLevel() > 29:
    LevelScaler = GetLevel() - 26
    if agi >= 100:
      LevelScaler = LevelScaler * 5
    elif agi >= 90:
      LevelScaler = LevelScaler * 4
    elif agi >= 85:
      LevelScaler = LevelScaler * 3
    elif agi >= 80:
      LevelScaler = LevelScaler * 2
    tmp_bonus = LevelScaler / 4
    bonus = bonus + min(12, tmp_bonus)

if GetRace() == IKSAR:
  bonus = bonus + min(35, max(10, GetLevel())

if IsMage():
  bonus = bonus + GetSkill(SkillDefense) / 2
else:
  bonus = bonus + GetSkill(SkillDefense) / 3

if GetAGI() > 70:
  bonus = bonus + GetAGI() / 20

if IsMage():
  bonus = bonus + TotalEffect(SPA_AC) / 3
else:
  bonus = bonus + TotalEffect(SPA_AC) / 4

if bonus < 0:
  bonus = 0

if IsClient():
  hardcap = 350
  if GetClass() == WARRIOR and GetLevel() > 50:
    hardcap = 430
  if (GetClass() == PALADIN or GetClass() == SHADOWKNIGHT or GetClass() == BARD) and GetLevel() > 50
    hardcap = 403
  if (GetClass() == RANGER or GetClass() == ROGUE or GetClass() == MONK) and GetLevel() > 50
    hardcap = 375
  if bonus > hardcap:
    bonus = hardcap

return bonus

That means you could have 12.75 Mitigation AC and 53.33 Avoidance AC from 255 Agility. Compared to 75 Agility where you get 3.75 Mitigation AC and 36.67 Avoidance AC.