Okay for one thing, I would choose more reliable mobs. As you level, the earlier mobs begin to take more damage from you (at 42, my poison on Ledise mobs does 104 ticks, whereas to temple mobs I do 84, a 20 point gain. I also gain roughly 20% damage vs Ledise Mobs on base damage.
I believe the level jump effect is obscuring your results, and I am too lazy too level a Sorc and do it myself. If you were to repeat your experiment with, say level 26+ mobs (like Unknown forest, I think that damage handicap will dissapear, making your results more reliable.
It is possible that is not the case, but just try it
BTW; you don't divide by the DIFFERENCE in magic attack (956 - 183); You divide it by the totals (183 and 956).
Remember, the amounts you are subtracting to divine your magic attack based damages are the Nominator; The Denominator would be the WHOLE Magic attack.
That being said, I find your Damage of fire on Mobs of low level to be laughable, indicating I think Sorceress damage has some serious issues.
-SM basic skills with 917 (using no bruiser)
Unholy Spear ~350-360
Dark Circle ~ 420-450
Dark Scourge ~ 530
Death's Cauldron 2 ~670
Night Terrors ~ 530
Poison Arrow ~ 295-310
Poison Cloud ~ 300-320
In other words, my damage averages are much higher, and my ticks are twice as fast. That's rough.