There is no need to turn a test bar or to have one perfectly centered. Rollie's Dad's method will work to adjust the tailstock center.
For those who are not familiar with Rollie's Dad's method, it only requires a rigid test bar uniform roundness and, preferably, diameter. If a bar is mounted in a lathe chuck and the surface near the headstock is swept with a DTI, the average of the minimum and maximum readings will be the spindle axis position plus the radius. Runout is automatically compensated for. If the carriage is then moved to the far end of the bar and the process repeated the average of the two readings will be the same if the carriage ways are parallel to the spindle axis. This process is used to align the lathe bed to the spindle.
For adjusting the tailstock, a slightly different approach is taken. the test bar is mounted between centers. A lathe dog and face plate are used to turn the bar. The minimum and maximum readings are averaged at each of two positions. If the tailstock is properly aligned the two averages will be the same. Any runout is canceled out in the process so the center drills do not have to concentric with the bar diameter. The tailstock can to be moved correct any difference without having to go back to check the headstock readings although it would be wise to do a final verification.
If the headstock center is known to be true (no runout), the bar can be rotated by hand, no dog and face plate required.