help with dosing MAG

Tank is 16 months old. I just started testing mg two weeks ago and seen it was low and decided to fix it before I get my stonies

 

Either way, if it's been depleated for a while, everything is going to absorb it for a bit.

 

That makes sense.

 

ok so i have been dosing the recomended stock solution for a while now and the mg wont budge passed 1200 and if i skip a day and wait 48 hours to test it actually drops in mg. im thinking this stock solution of 4 teaspoons at 50ml a day isent going to cut it. i used a reef calculator and put in the numbers to raise it 70ppm so i could figure out a dosage and its saying to add 94.8g's of solution to raise 70ppm. so could i follow this or stick out the small dosage of what the bottle says of 20gs at 10ml per 20 gallons? i feel like im getting no where with this. when i dumped 40g's of an "overdose" i had no issues and no high rising in any levels so it makes me wonder if im doing it right? help please

also using this calc; http://reef.diesyst.com/chemcalc/chemcalc.html

and selecting magnesium chloride anhydrous; is this the correct product since they dont have the brightwell magession to be selected? ingredients on the bottle says magnesium chloride and sulfate..