I prefer to not supplement with fiber due to my inflammatory bowel disease, so I take 300 mg magnesium citrate in the morning with breakfast, then an additional 300 mg magnesium citrate with dinner.
My suggestion would be to start with half that amount, see how it affects you, then increase as needed. Despite eating vegetables with all meals, I also struggled with constipation. Adding the magnesium has worked well for me, but I acknowledge we're all different.