Here is what i think that Lucky Charm only helps you get better drops on normal mobs, by my conclusion it doesn't help whit yellows at all, my opinion is that they don't stack one whit other, it says on stone "Item Drop Chance incrised by 15%" and on lucky it says "Drop rate by 50%" so thats 2 different words if you ask me Drop rate, Drop Chance. But im not the smartest 1 so i would like to now what others think about it.
Hope this helped.