Looks like Nitrogen deficiency, or maybe simply an old leaf from which the tree is re-absorbing the nutrients before dropping it.
Definitely not Magnesium; the symptoms are reversed. With Mg deficiency you would have green in the center, more or less in the shape of a Christmas tree, with yellow edges.
Have you recently moved your tree to significantly different light conditions? Meyers are quite sensitive to that and will often drop most or all of the leaves.

Aside from the gross unreliability of such home test kits, a ratio 2-2-3 NPK really doesn't work for a Meyer... especially one bearing fruit. You need a ratio of 3-1-2 and plenty of it; so you have essentially confirmed my first hypothesis, that your Meyer is suffering from lack of Nitrogen. Also, if you happen to be overwatering, it makes the shortage of Nitrogen worse. If it were my tree I would harvest your largest fruits; they are certainly ripe and are stealing nutrients from the new fruits.

Thanks for the help. I well reduce watering, and am going to switch to Miracle-Gro all purpose plant food in 24-8-16 and see what happens...

Better, but I'm not a fan of anything MG; if you read the label carefully, you might see it falls short in CA, MG. Many here like Foliage Pro; but I am not a container grower. My mature producing Meyers in the field get 3 pounds per year of 18-6-12 with 2% each MG and CA, plus 1% each Iron and zinc... it is specially made for me; so unless you need 10,000 pounds or more per year, you proly have to look for something close to that. FYI, I have found it nearly impossible to over feed a Meyer; but mine are all inground. Do a search here for Patty (hoosierquilt) or Mike (Meyer_mike) and you will get better advice for containers.

How long has it been in that pot? If years, re-pot. Better yet, plant in the ground where life is more stable for the tree.