I just keep score of my own drop rates. even if they publish drop rates you will not experience that exact percentage because you are not buying every single premium box/pack sold in the game.
the real world results will be some people experiencing higher and lower rates that averages to the total rate.
In short, write down your own buys and legendary drops to get a notionally accurate rate. Besides how would you validate a rate Gato told you without your own data set?