Oh yeah thats right man, as far as delta goes, if you check out the above post where the 20% mark was mentioned, you see at the bottom it says between 16 and 20 percent for deltas is mentioned. check out the CG on the CAD pick on this fourm for a quick delta refferance.
The whole CG calc is on the EDF fourm.
Compound wings are harder and Deltas have some considerations so thats why the mark is 18% or so. the MAC is the average chord or width of the wing, so if the tip of the wing is 3 inches and the root is 9 inches the average is 3+9=12 and devide by 2 to get 6. So 6 inches from the trailing edge of the root or 3 from the leading edge of the root is the aveage.
now take the 6 X .20 (20%) and you get 1.2. Measure 1.2 inches from the Leading Edge of the Root rib and thats the CG.
I like to use MM in the calcs cause its easier to use than trying to figure out what fraction 1.2 inches is