If the bar were 30 degrees off, and you zerod it , it would still be accurate. Because they were both reading zero at the same spot. Spin it around to face you and they should still be reading zero if in tram.
Maybe I need another set of eyes on this, because I thought the purpose of the bar with two indicators was there is no need to spin. The process is to have both indicators read the same when the head is trammed in. i.e. both indicators read the same, whether it be 0, 10, 12.5 etc. To make sure the indicators and bar are accurate you would need to tram the tram bar setup prior to tramming the head. If we go with the bar being accurate then all is well, but if the bar is off a half degree, the head will be off as well. People who design things make it seem difficult so they can sell their product.