The CG has to be in front of the point where the lift of the wing is centred. This is roughly at the thickest part of the wing. If the wing is straight then it's easy to figure. If the wing is swept so the high point varies from root to tip then a bit more guesswork is needed. I'd go for maybe 5cms in front of the wing high point to begin with but be very cautious on the first flight while you figure out how it's reacting. If the CG is too far back then you're guaranteed a very interesting first flight