My magnesium level is:0,64 mmol/l (ref: 0,75-1 ).i`m taking daily 250 mg magnesium,but it seems to be not enough.how much is recommended in this case?

Review with your doc: Magnesium is one of those minerals that can cause a lot of problems if you take too much of it. The recommended dietary allowance of magnesium is 320 mg; getting this from food sources is entirely safe. Taking more through supplements may cause diarrhea, nausea/vomiting. I'm not sure why you think 250 mg is not enough, but taking up to 320-350 per day should be safe. But check with your doctor:)