Yes, it is confusing.
This on-line calculator may make it easier for you:
http://scherrer.pagesperso-orange.fr...glish/mce.html
This link will explain the concept better than I can:
http://adamone.rchomepage.com/index5.htm
What I did was to divide the area of the half wing into two regular shapes.
Then, determined the center of each area, where the MAC is.
Then, I found the point located at 25% of the LE for each area, where the CG for each area should be located at.
Lastly, using the formula included in the schematic, I found a mean location for the CG of the total area, respect to the LE at the wing root.
Once you have the total MAC, you could use any on-line calculator, such as:
http://www.geistware.com/rcmodeling/cg_super_calc.htm
In any event, the 80 mm seem to be too forward a location for the CG of your plane, and the way the plane flew shows nose heaviness.
And, yes, heavy on the nose planes load the wing more, and tip stalls show up.
This thread discusses tip stalls' why and when:
http://www.rcuniverse.com/forum/m_89...tm.htm#8954798