It might be convenient to zero your Z axis at the center height of the A axis centerline. However, the actual zeroing of the A axis would mean finding some kind of switch home on a cnc controlled A axis. This would be a built in function of the controller for the A axis.
It is also possible to offset the start position of the A axis with a degree entry in the relevant work offset table. If your part has an identifiable feature on it that you need to indicate into position, then this feature will have an angle that you should know from the print. Note the difference between the A display current position and what you intend that the current part position should be at, and enter the difference in the work offset that you will be using.
On a manual A, you just turn it to 0 degrees to start.
Not sure if that helps or not.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)