You're on the right track. The kit plans are likely as not just conservative.

The other way is to trim the model for level flight at about 1/3 throttle. Then tip the model over into a 45* dive and watch it for a moment. A forward CG will make the nose lift back up. A CG that is behind the neutral point will make the model tend to tuck to a steeper dive. When it's right on or very close to the neutral point then it'll hold the same dive angle until you pull it out.

Remember that the CG position and elevator trim required to hold that level flight point are both related. The more aft the CG the more down(or less up) the elevator.