>Just wondering what would be an accepted concentration level of pellets to water<
I suggest you take some water from your tank. Test for ammonia, then add enough of your magnesium supplement for an increase of 100 ppm Magnesium. Then test for ammonia again. If you see a significant difference in the test (a boost of 0.5 ppm ammonia) then you might want to either reconsider the use of the mag flakes/pellets, or plan on adding the supplement slowly enough so that your ammonia levels do not spike.
As for calculations, the magflake (and the pellets I believe) are magnesium chloride hexahydrate, so they are about 11.8% magnesium. So if you have a liter of tank water, you'd need to add 847 mg of the magflake/pellets to get a 100 mg/l (almost the same as 100 ppm) increase in magnesium. If you don't have an accurate way of measuring that low of a weight, just increase the volume of tank water until you can measure the amount of magnesium you need for it.