There is some coorelations between seatback angle and Kx.
Exact measurements require a swing and precise knowledge of the position of the center of gravity.
A very good estimation for the position of the center of gravity is the riders belly button. The bike swing is used to determine Kx for the class project bikes. The radius of gyration is an important value for the designer. Measurement may be too difficult for most builders. Kx is .44 meters for a bike with a vertical seatback and feet lower than the seat bottom.