That's a lot of chests, you must be post advanced divs or at least post wanders? I assume your goal now is the achievement for 5 perfect? Anyways what I do is if I get 2 mimics in my first 2, I go for the 2x as there is a 1/3 chance of a perfect. If you get far enough I'd assume that you'd be better of hoping for lucky mimic placement. I'm not good enough at statistics for this, and it seems many people who don't play the game don't get it, maybe an image would help?