Of that magnesium oxide 440mg about 8 mg gets into your blood stream. It is useless for blood levels. Studies have demonstrated this and are publicly available. 8mg is what is in an egg for example.

It would be best to try a better version. If you increase this product you would be getting too much B6. So don't take two of them etc.

Basically there is NO mixed tablet supplement that I have ever seen that provides adequate magnesium. They all use the old OXIDE for cost reasons, and historical reasons and to keep the tablets small. Ionic Fizz products, that you mix in water, DO have a more balanced amount but they are not tablets.

If you want to keep your supplement and use it up, add on SlowMag twice a day, and see what happens within a week.
Two SlowMags provide 1/2 the RDA of magnesium.

Or you can eat high magnesium containing foods. Almonds are quite high, and 3 oz of unsalted give 270mg!

Avoid OXIDE forms of magnesium, they do not provide much besides a laxative action.