The location of wheels for conventional geared model aircraft should be in line with the actual CG, and swept forward 15 to 18 degrees, depending on the surface that you fly from. Off of smooth pavement, 15 degrees works well, but on grass you want about 18 degrees. The length of the struts need to maintain prop clearance with the tail slightly above a flying angle of attack, unless you want to always do three point take-offs and landing (hard to do). If the gear are too long, you end up with too much angle of attack for the wing, which makes take-offs more difficult, so having a longer strut on the tailwheel helps make ground operations easier.

The CG is not where an airplane balances on the wing. It is the actual location where if the airplane is suspended from any point, the CG is directly below the string. Done from multiple locations, you soon find out where the actual CG is. Of course, after you hang on the landing gear and wheels, the CG will be shifted.