Mr. Bonk,
The short answer to how much reflex you will need is......."It depends on your CG."

On a flying wing with a symmetrical airfoil, the reflex needed in the elevons is entirely dependent on the CG location. The more nose heavy your King Kombat is, the more reflex it will need to maintain level upright flight.....AND the more control surface deflection it will require. The further aft you move the CG, the less reflex it will need, and the less control surface movement it will require. In fact, if you move the CG location back far enough, you will eventually find a "sweet spot" for the CG where no reflex is needed at all. And with the CG at the "sweet spot", the plane will fly straight and level upright, and also straight and level inverted without having to retrim the elevator. For combat, this "sweet spot" CG works great! It gives you maximum maneuverability, and the plane simply goes where you pointed it last.....no matter whether you were upright or inverted.

For your initial test flights, I'd recommend setting the CG at the front edge of the spar. Then adjust your linkages to get about 1/16" of up reflex in the elevons. This should get you pretty close for the initial test flight. Once it's trimmed out and back on the ground, be sure to note how much reflex was needed for your particular CG location.

To handlaunch the King Kombat, have a helper grab both wingtips, rev your 40GP to about 1/2 to 2/3 throttle (not full), and give it a gentle push at an upward angle of about 30 degrees. The partial throttle launch will quickly get you up to flying speed, but also allow you some extra time to make any needed stick corrections. Once it's been trimmed, handlaunches are much easier. It simply goes where the handlauncher aimed it.

And after the initial trim flights, you can easily handlaunch it yourself. Using the left hand, wrap your fingers around the leading edge next to the fuel tank, and place the thumb across the top of the motor mount. Set the throttle at about half power, and give the DaWing a slightly upward and forward push before releasing it. Once properly trimmed, it will fly right out of your hand.

With an OS 40FP, no muffler, and an APC 9.7 prop I once clocked a DaWing at 117 miles per hour, or 188 kph. I would expect a King Kombat with a Magnum 40GP be similar in performance.