The probability based approach would be 1 - (1 - 0.2) * (1 - 0.3) = 44% dodge rate. However, I suspect that only the highest dodge instance applies, which would be 30% here. Potions are a special case, since you paid for them they stack, using the probability based approach.
I have an answer for you!
Each chance is calculated separately. They do not stack.
Okay, then total percent of dodging is 44?
If the two chances are indeed independently calculated, then yes. However, that 44% is not a number stored in the code anywhere. It’s just the theoretical outcome of individual 20% and 30% chances.
(If you want to test this out and take some data, I am all ears. Much too lazy to do it myself.)
Sure, this figure is just result of probabilistic formula.