It doesn't matter where the bellcrank is with respect to the CG or thrustline.
It does matter where the bellcrank is with respect to the leadout guides, and where the leadout guides are with respect to the CG matters a LOT.
The leadout guides need to be behind the CG -- probably about 3/4" for a normal sized airplane. See if you can scare up a link to Line III on Google. Download it, install it, and use it to figure out your leadout position.
Up and down, the leadout guides need to be pretty close to on the CG. This probably doesn't matter as much in scale as in stunt, but you're modeling an airobatic plane; if you're going to do any negative-G maneuvers as part of your scale flights then you want the vertical position of the leadout guides on the CG (if you don't, the plane will be rolled in, either upright or inverted, depending on whether your leadout guides are too low or too high).
Once you've got your leadout guides figured out, you want the bellcrank positioned to minimize the amount of bend at the leadout guides, without making your construction impossible. Mostly you're trying to minimize friction at the leadout guide. On that plane, placing the bellcrank so it never gets closer than about 1/2 or 3/4 inch (assuming a 4 or 5 foot wingspan) to the leading edge is probably good enough, although if you feel like being fanatical you can try to get it further forward.