I take 3 x 500mg magnesium aspartate a day which has the equivalent to 37.4mg of elemental magnesium in each 500mg.
Magnesium citrate is a laxative and has a much lower amount of magnesium in it and is not absorbed into the body in the same way. Erhum, you are literally dropping the tablets down the loo
To get the best benefit of magnesium as a supplement try using either magnesium oxide or magnesium aspartate. But do be careful not to take too much as it is toxic if the recommended daily amount is exceeded.