Searching for the Back attack, preemptive attack chance rate formula in FF8

[Kitsune]

Hi!

I search an info about Strike back and preemptive attack in Final Fantasy VIII (if possible PSX version but don't know if it change about the version)

I have found this formula in a document on the web:

Code: [Select]

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10.2  Back Attack, Struck First
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To determine whether you get an advantage/disadvantage over the enemy:

  rnd = [0..255] + EncounterMod
  if (rnd < 20) "Back attack!"
    else if (rnd < 236) no change
      else "Struck first!"

  Note: EncounterMod can be either 0 or 20, depending on the enemies.

  Probability of..
    "Back attack!"    20/256 or  0/256    7.8% or  0%
    no change             216/256            84.3%
    "Struck first!"   20/256 or 40/256    7.8% or 15.6%

  Note: Equipping the "Alert" party ability reduces "rnd" by 20, essentially
        increasing the odds of "Back attack!" and decreasing odds of
        "Struck first!"

Back attack:
All allies will start with 100% of the ATB bar filled.
Enemies start with 0% of the ATB bar filled.

Struck first:
All allies will start with 0% of the ATB bar filled.
Enemies start with 100% of the ATB bar filled.

No change:
All enemies' and allies' starting ATB bar calculated normally, see section 3.1
for more details.

Source: https://gamefaqs.gamespot.com/ps/197343-final-fantasy-viii/faqs/58936

I search for the EncounterMod variable, it must be either 0 or 20 but I don't find anything on the web about it and monsters or battle scene.
If you have an answer? Thanks.