Mark, when measuring the size of your tank, measure the inside of your tank, not the outside. Makes a big difference sometimes in the total volume. Then use the
Aquarium System Volume Calculator to calculate real volume. I think the calculator adjusts for rock volume as well. There is also a place to add in the volume of your sump:
http://home.comcast.net/~jdieck1/volcalc.html
Once you have the right volume, use the
Reef Chemistry Calculator to double check your calculations. I don't use Seachem products, but Seachem's Mg products are listed in this calculator. When my Mg gets low, but never as low as your levels, I have to add a whole whack of of Kent's product:
http://home.comcast.net/~jdieck1/chem_calc3.html