250 mg is not very much magnesium. Most people need much more than they are getting.
I think transdermal magnesium is one of the best ways to get magnesium because it is absorbed directly through the skin. I use Magnesium oil (buy it by the gallon). I get the Health and Wisdom brand. I also take magnesium glycinate chelated capsules. Some people have a problem with large amounts of mag causing loose stools (I certainly do!). The chelated is buffered, which eliminates any caustic effect on colon activity.