You can most likely get it close enough if you use your fingers under the wing root at the point shown in the manual. I made a simple balancing jig by drilling two holes in a board a little wider than the fuselage, and putting a dowel with the end rounded off in each hole. I then marked the desired cg location on the bottom of the wing (you can use a piece of tape, or a magic marker, etc.) and balanced on that. I’ve used the same jig on several models with good results.

The ruler under the fuselage, while good in theory, is far too sensitive. Because the cg is well above the ruler, it will fall one way or the other unless it is perfectly balanced. For a trainer with dihedral, the fingers under the wing tips is not nearly sensitive enough. Because the cg is well below the tips, the cg can be off a good bit and the model will still hang pretty level. With the support at the wing root, the sensitivity is about right.

If the cg is not pretty close, try to move it by moving the battery rather than adding weight. If that doesn’t bring it in, adding a little weight won’t have much effect on performance of a trainer.