Test launch it without the engine running.

Move the CG forward until is always dives on launch. Then, add up reflex to the elevons until it will launch into a stable glide.

Or:
Set the elevons at 3/16" up from 0. Then, move the CG forward until it launches into a stable glide.

Either method will give you a starting point from which to adjust the CG and reflex to suit your flying style.

HTH,
D