Raising Mg

[Johnny Reefer]

Just lookin' for opinions on this.
I'm using SeaChem Advantage Mg to raise my Mg from 975 to 1,290.
The jar says that 5 grams will raise 21 gallons by 5ppm.
My system is 205 gal net.
205/21=9.76. Right?
9.76 x 5g= 48.8grams. Right?
1,290ppm - 975 ppm = 315ppm. Right?
315ppm/5ppm = 63. Right?
63 x 48.8grams = 3,075 grams. Right?
This seems like a whole whack o' Mg to me.
I have to dose over 3 Kg's of Mg to get up to 1,290ppm???
Does this sound right?
Anyone else ever have to use so much Mg, initially?
(Seems like I'm gettin' a calculation wrong here?).

Thanx much,