I take a variety of supplements.
As far as magnesium, I've changed how and what I take over the last 10 months as I learned about it.
I started out with magnesium oxide also. I didn't know it was such a poorly absorbed form. When I learned that, I couldn't buy more at that time and wanted to use what I had bought so I started taking more so that I absorbed more of it. I could tolerate 500mg twice a day with no problem so I also took 250mg mid day and sometimes took 750mg at night if I had constipation which I did occasionally. It never gave me an upset bowel, so I could've tolerated more.
But then I magnesium citrate. I was taking 400mg twice a day with no tummy issues. 800mg total.
Most recently I bought magnesium carbonate in powder form. I mix it (400mg) with water and ascorbic acid (3000mg vitamin C). I throw in a little sea salt and NuSalt (potassium), a squeezed lemon and a few drops of raspberry stevia and it's a really good drink. I'll have that twice a day. Since I have no digestive response from that dosage I may add a third drink a day. It's helping me get more water too, which is never a priority for me.
Besides that, I take krill oil, MCT oil, collagen and ginkgo biloba.

If you're interested in learning more about magnesium and how important it truly is there's a great book called The Magnesium Miracle.

I take 800mg Mag Glycinate at dinner (used to take it at bedtime but the water needed to swallow the huge pills had me up to pee in the night haha). I'd started with 400mg and honestly don't recall what made me double it, but I find that with taking 800mg I sleep better than I did at 400mg, and since I am for the moment stuck taking narcotics regularly the 800mg dose (plus coffee and occasional miralax when needed) keeps things moving pretty well on the back end. I think I ought to mix up my Magnesiums though, as the Glycinate is rather expensive to be using that much just to achieve the laxative effect!