I recently ran Axis Calibration "Set Steps per Unit" on my Pro4824 after making some changes on the machine. I use a 48" steel ruler and calibrate to 46" on the X axis and 22" on the Y axis. You need to run the calibration over the longest distance possible for accuracy. I use a pointed centering bit for accurate measurement.
After calibration, I cut 18 2.05" disks out of 3/4" plywood to fit inside 2" PVC pipe for a project I'm working on. Each one measured exactly 2.05" and fit perfectly inside the pipe. I then cut 2-7.75" disks and 8-2.28" disks. Each one measured out perfectly.