Need help figuring out something

[patpare]

Ok I guess I really should have stayed in school instead of doing what I was doing!
Anyways I'm trying to figure out how much Reef advantage magnesium to add to my system to bring it up to 1350 from 1200.My tank has a total of 40gallons of water including the sump.

Here are the instructions

ADVANCED: Check magnesium level, then follow addition regimen above until magnesium is adjusted to 1200–1350 mg/L. Each 5 g/80 L will raise magnesium by about 5 mg/L. Amount or frequency can be adjusted, but do not exceed 25 g/80 L per day. Thereafter, use as required to maintain magnesium.
Thanks for the help