You should have no problem with 200 mg/day. I know a person who, as a scientist, has been highly methodical about controlling his own Mg intake. He says he's a 'mg dumper'. He goes through magnesium like we do water, and pees it out copiously. So, he literally mixes a liter of magnesium water and drinks it each day. Sounds bizarre, too bad to be true, but I know him and he's a regular over at afibbers.org forum. Highly credible. So, how much does he ingest daily? Over 1000 mg!
That's magnesium. I have not researched a safe daily dose of potassium. You should.....SHOULD...get enough in your diet (beets, figs, pistachios, potatoes, oranges, kiwi, etc), but if you aren't, a few mg supplementation with shake-the-bottle potassium chloride salt (your grocers should have it) will suffice. Adults need about 3.5 gm a day...a lot!