I went out yesterday and confirmed that pets/avatars/monsters use either exactly the same fSTR equations as players, or very similar equations. I chose to use avatars for this testing because their base damage is lower (so the monster lasts longer). Basically I went in to Abyssea and used celestial avatars with varying amounts of +STR from Atma (0,10,20,30,40,50,60,70,80,90,100). I targeted EM Bluffalo because we know their VIT (/DEF). There is a very obvious frequency spike at 1.0 pDIF, so that lets me know the avatar's base damage (lowest number in this frequency spike).
My results match the fSTR equations for either D48/104STR or D47/108STR. They do not match for any other combination. I could tell the difference between these two cases if I was capable of adding 1~4 or 9 STR to my Avatar (or reducing the monster's VIT by 1~4 or 9). I should be able to use Choke with my 85 INT mule to lower the Bluffalo's VIT by 9 and figure out exactly what the D/STR combination is.