Measure back from the center of the leading edge of the wing itself (or it's about 1/4 to 1/2 inch in front the hole in the center). Don't worry about the incidence- which, you're right, the leading edge is 20 cm higher than the trailing edge.

To get the CG right on this (or any other) plane, adjust the battery back and forth so the elevator is level.

However, lucky for you, the Soarstar's the elevator is glued flat on the boom, so just balance it so the boom is level.