Kev, I measured the inside at the center of the dash, 46 3/4 inches top to bottom. There is plenty of room to get to the top edge and mount the magnets, if you have long arms! Lol. Sorry I didn't get better measurements for you the first time, didn't understand how the shade mounted.
Rather than using 3M tape, which I love, but will let go in high heat, I would use clear silicone from a fresh tube. I've used silicone to mount several things on windshields and never had it come loose, plus it will put the magnets physically closer to the glass. On our last boat I mounted a medium sized compass to the windshield using silicone, boats can really take a pounding in rough water, it never even wiggled!
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