Easy and accurate C of G calculation:

Place model nose against wall, measure the distance of each wheel axle, main and nose, to wall. (you can use any other datum point you wish but a wall is often convenient) Place scales under each main wheel in turn and pack up nose so that the machine is level, note weights.

Then measure weight on nosewheel.

Multiply TOTAL weight on main wheels by distance from wall or other chosen datum. Moment A.
Multiply weight on nose gear times distance from wall or datum. Moment B.

Add A + B to obtain total moments.

Divide value of A+B by TOTAL weight and that will give you exact distance of C of G from wall or chosen datum. eg nose.

To adjust, I tape a paper cup to nose or tail and add fishing weights to get C of G exactly correct.

Easy , quick, accurate and free, can easily be done by one person, and prevents hernias. Works on 747s too, (which are NEVER inverted !) honest !