My mg depletion rate is 100ppm. Is it possible?

[Coleus]

I have 155 gallon DP with 30Gallon Sump. Have about 200lbs rocks and have fair amount of lps and fishes.

I am having a hard time to stabilize my mg. I tried to test week after week and it seems that my depletion rate is 100ppm, my calc and alk stabilize at 440 and 10 with 2 parts dosing.

Does it seems high? I use Reef Chemistry Calculator and i need 672 grams of MagFlake (got from littlesilvermax) per week. That seems to be a lots


Your thoughts?