magnesium may well cause a upset stomach in the beginning, but generally if using the malate form it should be well tolerated up to 600 mg. I dont take all at once though.
I know vit C tolerance differs quite a lot; mine is maximum 3-4 g
So I have no other suggestion than to find your levels;
I upped my magnesium the other week as I tried a new supplement, and boy did I sleep well even if I exedeed my bowel tolerance (1000mg)