I don't know if this will make any difference to what you are trying to do, but...
I just plugged your numbers in Chuck's fert calculator and I got 3.53 ppm Mg. http://www.csd.net/~cgadd/aqua/art_p...osage_calc.htm
This is what I did:
Amount to add: 0.25 tsp (the calculator converts ¼ tsp to 1.35 grams)
Amount of water to mix with: 1 mL
Tank Size: 10 gallons
Each ML of this solution that you add to your tank will add: 3.53 ppm Mg
Suggested Target Level: 5 to 10 ppm Mg
Chuck's calculator uses MgSO4.7H2O for magnesium sulfate. Chuck says that 1 tsp is 5.4 grams or 1.35 grams for ¼ tsp.
The ppm difference is that you used ¼ tsp = 1.0 gram instead of 1.35 grams.
Are you using Greg Watson's Plantex CSM+B? He adds an equal part of magnesium sulfate to his trace when he mixes it. How To Dose Plantex CSM + B