probably works like this
https://pathofexile.gamepedia.com/Scorching_RayBurning Debuff can have a maximum of 8 stages
Debuff applies -3% Fire Resistance per stage
Total Fire Resistance penalty from all Beams cannot exceed -24%
if the monster started with 0 fire resist, and you 2hit it, then this would be a 1.5%more damage*0.25chance=0.375%more damage
vs a boss, it would be 24%more damage after 32hits
some monsters have over 100 resist, this would do 0%more damage