Using OBC with a Taig lathe, the best way I know to get an accurate diameter is to pause the job after a first turn pass, measure the diameter and then change the respective coordinate value to the measured diameter and then continue the run. But the software does not allow this. My suggestion is to allow it, perhaps with an appropriate warning. Otherwise I have to stop the job, change the value and restart the job from the beginning.
Thats correct. Pause in grbl does not hand control back to the user (too risky) Stop, and resume from line only way
Split the job into two in CAM - one program to rough out and give you a diameter to measure and a second for finishing that you run after fine tuning the diameter.