magnesium test kit ?

Yes, you are correct. I don't use a math formula, I just go by the chart they provide but that sounds correct, 0.07 mL left would be 1395.

Just a hint for that kit, the color changeover can be a little difficult to see sometimes. It's not like the Calcium kit that goes from pink to purple then definitively blue, or the Alk kit which turns from blue to purple then pinkish-orange then pink.

Sometimes it will change over to a steel-bluish-gray color, and won't turn definitively blue, other times it will turn definitively blue. I think this has something to do with the form of magnesium in the aquarium but that's just a total guess. Ever since I started using BRS 2-part and using their magnesium, I get consistent results where it goes from that purpleish color to the grey-blue and then a light royal blue. I find that right after a PWC using Reef Crystals it will start out more of a deep magenta color and then the color change-over will be to grey-blue rather than light royal blue.

Also I think it helps to swirl for more than 30 seconds after the drops (maybe 45), and about 30 seconds after the powder (versus 10).

Hope this helps. More info than you asked for...