The actual magnesium you would be getting is the elemental magnesium (100mg). But not all magnesium supplements are created equally. Magnesium malate is one of the forms that is recommended for fibromyalgia. But what really matters is HOW MUCH elemental magnesium is being absorbed and getting into the cells.
What we are learning about supplementation is it is not so much the amount that is listed on the bottle, but how much is actually bio-available and is being absorbed by the body. The rule of thumb of magnesium optimal daily value that I've seen is around 500mg-1000mg . Now has this changed since we've got magnesium in different forms that are more bioavailable? Probably. But if one is deficient, it will take longer to reverse at lower doses. Also, the body can only absorb so much at one time so taking it multiple times a day is better. Do you have classic magnesium deficiency symptoms (agitation and anxiety, restless leg syndrome (RLS), sleep disorders, irritability, nausea and vomiting, abnormal heart rhythms, low blood pressure, confusion, muscle spasm and weakness, hyperventilation, insomnia, poor nail growth, and even seizures)? Then your requirements would be higher than someone who doesn't have these problems. You may find yourself trying different forms of magnesium until you find one that doesn't seem to go thru you and seems to work for you. I use either magnesium aspartate or magnesium taurate along with a chelated multi-mineral to get my calcium to magnesium ratio to 1:1. I've not tried magnesium malate in many years and honestly don't remember if it was helpful as I was taking for fibromyalgia (that was really undiagnosed hypothyroidism). I've tried magnesium glycinate but they were B-I-G tablets and were just too hard to swallow.
In the end, you might find blood tests to be helpful in finding out where your magnesium levels are and if the magnesium malate is doing what you want it to. Janie has a really good write up on magnesium (and blood test to get) on her website: http://www.stopthethyroidmadness.com/2010/05/19/magnesium/#