well the first thing that strikes me is that magnesium isn't required to be on labels. and if it isn't required to be on labels, there are a lot of companies that aren't going to waste the ink printing it on there.
so if you really want to know how much magnesium you were eating in the first place, you need to contact all of the companies about all of the products that you were eating in the first place to get the full nutrition information for their product. you may be able to find more detailed info online, but you might have to send out a lot of emails. then you need to adjust all of the info you're using with the new info as you get it. then look at what you're actually getting.
alternately, take a look at the spark page on magnesium. are you eating a fair amount of food from there? yes? i think i remember dietitian becky saying that for the nutrients that aren't required to be on labels, if you're eating a fair amount from the list and you're tracking at 70-80%, you're likely getting all you need, it's just tracking errors [ie not listed] that are preventing you from seeing what you're actually getting.
-google first. ask questions later.