Let the math begin lol.
The chances is considered before the added effects, so the increase in chance will be of the chances of not poisoning with Poison Jab.
>Chances of not poisoning with Poison Jab = 1 - 30/100 = 70/100
Increase in chance = 70/100 x 20/100 (the added effects of Poinson Jab) = 14/100 or 14%.
>Total poison chance = 30/100 + 14/100 = 44/100 or 44% chance of poisoning.
Hope I helped!