This is based on the picture from the website, so it may not be accurate with the final version of the Model X. Hope that helps. I'm off to go measure my garage!
IIRC the absolute height has been estimated at ~84-88" previously, so you're probably about right. It's worth noting that Elon confirmed (at TESLIVE?) that there will be sensors built in to the door which will stop it a couple of inches short of the ceiling/obstruction.
There is an assumption that there cars were photographed next to each other. Reflections say otherwise. These pictures may have been taken months and hundreds of miles apart. The Photoshop artist juts made them "look good" next to each other. This one is likely a composite as well.
They're likely just renders, but I would assume the same camera/scene setup was used (no point in doing it twice). Also, they can alter the design before production, so these numbers are subject to change. The "guaranteed error-free" in the image is just a little tongue in cheek