Slappie,
You got it right....
I've gone as far back as 20% on my wings..so, if you have poor elevator response you may be a little nose heavy.
I average about 16-18%...
which if calculated on your straight wing of 7 inches
7 x .16 = 1.12 inches from the leading edge
7 x .12 = .84 inches from the leading edge
So...nice estimate would be to balance it right at an inch from the leading edge and go....
Don't forget to have a couple degrees of reflex in your elevons (both slightly up).
Wings like to have a little reflex...
I've done the same thing as Tall Paul has before....funfly airfoils work great at flying wings....
Good Luck!
CrazyHerb
www.crazyherb.com