Opponent's ninjutsu doesnt affect poison damage, neither does your ninjutsu. Say if you are using poison fog with water main, the damage when the mystery is used is accounted by your ninjutsu and opponents resistance. At the end of the round, when poison damage is ccalculated it's solely based on resistance of each of the individual ninjas in opponent team. I think kakashi clones have 0 resistance and you get 1999 poison damage for one level of stacking and max stack on them is 7996(1999+3*1999). I didnt understand how you ended up assuming that ninjustsu affect end of round poison damage. Can you share your experimental data since you have mentioned experimentation in a controlled battle. Resistance matters since if you see the poison damage isn't same on different ninjas of your opponent since they have different resistance values.