For Mg I went with what I had. Salifert and Red Sea Pro. I've always went with Salifert here as they were made for detecting aquarium saltwater Mg levels and it has always been very consistent for me. It also matched up pretty well against Triton in my last triton review.
Red Sea Pro Mg test. I HATE. Out of all the test kits I've every used I hate it the most. I've had nothing but unreliable and inconsistent results with it. I have tried multiple reagent refills and even two new test kits to make sure the measuring devices were not off.
Salifert: 1470-1485 (I keep meaning to back off the Mg I add to my fresh salt mix but hopefully soon it'll get depleted with the new corals being added too)
I find it easy to see the end points. Some do not.
2nd to last drop which would be ~1440-1470
Last drop put in to be at .01-.02 or ~1470-1485
Red Sea Pro: 1100 "Curse it and crush it! We hates it forever!"