The problem you might run into, is that if the diameter changes, you'll need to change the motor tuning to match the diameter.
I'm assuming that your A axis is 350 steps/degree? If so, then you have 126,000 steps/revolution.
So your steps/inch would be (pi*diameter)/126,000.
You might find it easier to create 2D code, and use CNC Wrapper to right code to use your A axis. CNCWrapper - Home Page