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]

-------------------------------10.2  Back Attack, Struck First-------------------------------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 3.1for 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.