The short version is to get the rack at same height as the tyerod attachment point, thta way they rise and fall at the same 'rate'. A few different ways to get this done.
If the attachment point is higher or lower than the rack centerline then the arc travel will run 'uphill or downhill' and toe the tires in or out as it goes over a bump. The worst case is when the angle is uphill and when the tire falls it will toe 'out' and that is when the car will change direction, esp when cresting a small hillock. If id is downhill then it will toe in, you'll have tire scrub but the car will go straight.