For your King Kombat's initial flights balance it on the front edge of the spar. Set your elevator throws to 1/4" up and 1/4" down.....and set the aileron throws to 1/2" up and 1/2" down. As you move the CG rearward, it will take less elevator travel to stall it.....but the rearward CG will make it more maneuverable.

Back in 1979, several of us around here built and flew King Kombats. Initially we powered them with OS 25 bushing engines, but later changed to the K&B 3.5cc engines to get more power and speed.

We also discovered that if you left the wingtips off of the King Kombat, and used a K&B 3.5cc, the elevons would flutter in a slight dive. Since you are planning on using a Magnum 40GP, be careful because your King Kombat could easily go fast enough to flutter the elevons.

Eventually, our King Kombats evolved into a plane I called the DaWing, which used foam wings, OS 40FP engines, and had a modified elevon design that completely cured the flutter problem, even without the wingtips. In fact, I used to have a DaWing powered by a Rossi 40 just to go blazing around the sky. If you visit my DaWing web page, you will see the distant resemblance to the King Kombat.
http://mypage.yhti.net/~dmcdnld/dawing.htm