I've just realised, what you need to achieve may have to be via a combination of Macro and PLC.
The macro allows you to run normal G-codes, but I'm not sure if you can run calculations in a Macro.

What you may need to do, is run the Macro, which then moves to the PLC to calculate the rotation required, which is then stored in a variable the macro can access. The macro then carries out the required A axis movement.