We consider a spatial system of bars (with possible errors of fabrication, that is, some bars are longer or shorter than their nominal dimensions), connected one to another in a single point by a spherical joint and connected to the frame by spherical joints. The system is acted by a system of forces acting only at the common end that reducing at the common point at a resultant. After the acting of these force, the common point is displaced to a new position. We want to add a new bar in order to minimize this displacement (or, at least, some components of the displacement). The addition of the new bar must lead to a new deformation that has to be small enough in order to maintain the precision of calculation. Some details of analytical calculations are presented. The obtained formulas lead to the solving of a nonlinear equation or a system of nonlinear equations. A numerical example will highlight the theory. The influence of different parameters is also presented.