Ya know, I don't think any of the Mg test kits actually test over 1500ppm. However, you can still test for higher amounts. For instance, with Salifert, you just fill the titration syringe, all the way up, again, as if you were starting the test, and continue to titrate. Once you've reached your end point, you take your reading, from the syringe, and then add that reading, to the 1500ppm reading, from the first syringe, that you emptied.
Same can be done with Salifert's calcium test kit.
Does that sound a confusing to you, as it does to me, when I re-read it? lol