I taught wheel alignment for 30-something years, as well as doing a lot of round track race car chassis building. One of the primary causes of bump steer in an independent suspension system is an error in the radius of the tie rods in comparison to the length of the lower control arms. All the relationships of suspension components and steering linkage need to form parallellograms. This includes the length of the Pitman arm and the idler arm, and the length of the relay rod between them. Once the position of the Pitman arm and the idler arm is established by locating the steering gear and the idler arm mount correctly, then the inner tie rod pivot needs to be located as close to the lower control arm pivot as possible. With the wheels pointed straight ahead, everything needs to be square- - - -Pitman arm and idler arm pointing straight ahead or back, and the inner tie rod pivot aligned with the control arm pivot. It also helps if the tie rods are approximately parallel to the ground at normal ride height. The idea is to have all the linkage as square as possible while driving straight down the road, and have as little change in angularity or radius when negotiating turns or bumps.
Jerry
