OMGZ... 800ppm Mg is like majorly low. Pretty impressive that nothing has died!
Just get some epsom salt from Long's drugs or Safeway for now. That's plenty good, and inexpensive too.
I would go to 1200 in one shot, then 1400 in this case. I don't think gradual change is as important as getting out of an excessively low level at 800ppm.
Use the tank chemistry calculator:
http://reef.diesyst.com/chemcalc/chemcalc.html
For example:
If you have 100g total system volume, enter 100 in the top box.
On the far right, enter 800 for current.
Enter 1200 for desired.
On the dropdown, select epsom salt.
Click "calculate", and it tell you you need 52oz.
Dissolve that in about 3-4qts of fw, then add to your tank.
The next day, repeat the process to figure out how much to add to go from 1200 to 1400.
Use your specific system volume, not 100g