We temporarily installed the medicine cabinet to make sure framing was exact before tiling. Forget about measure twice, cut once. I’d rather check until I can stop being worried about it and sleep at night.
Plus if you check before it’s too late (meaning too expensive), you can fix things.
This medicine cabinet is actually 3 put together for a whopping 48" wide by 30" high. A fabulous product by Robern, it is mirrored on the inside as well.