The leadouts do not affect the c.g. position, but the c.g. does affect lead out position. C.G. is aerodynamically related for stability - depends on the aerodynamic center (a.c.) or "neutral point" of the whole model or roughly on the a.c. (1/4-MAC point) of the wing, usually placed somewhere ahead of it. There are a lot of more specific posts about where to place it on flapped vs. unflapped models of different proportions. Experience and reasonable rule-of-thumb approximations usually places CL c.g.'s at 15% -19% of the mean aerodynamic chord of flapless planes and 20% -25% of flapped planes. This is a bit forward of FF, RC, and full-sized aircraft positions. Depending on a plane's size, weight, speed, and line diameter (drag), the leadouts usually exit perhaps an inch behind this point, +/- some. You can compute this close enough to start, using on-line available programs. Trimming follows.

That should serve for a start.

SK