As far as I can see the trailing link issue makes no difference to the CG machine. The point at which the wheel/tire touches the transducer is the projected point of support of the airframe. In other words it is the supporting position of the transducer that matters not what it supports. To take this to the absurd you could put a solid trailing link 3 feet long behind the plane and still the point where the transducer supports that trailing link becomes the measuring position. This is all about moment arms, not just the weights measured. To simplify: if you have the main wheels' axles 2" behind the strut center then you would measure the length to the CG, nose wheel support point (axle), and weight addition positions, from the 2" behind the main strut position. I hope this helps...
Len
I recommend using a laser level and get the beam to pass by the contact point of the main gear tire on the scale.
The intersection with the laser beam to the longitudinal axis is the base measurement point.
The accuracy of this point will be the width of the laser beam. Usually about 0.5 mm.