The HFR reading from your AF routine is just a relative number and has no absolute meaning. When a frame is analyzed, the calculated HFR is a result of the star sizes used to make the HFR calculation. So your first AF frame might show an HFR of 3.5 and the final focus frame might show 1.1. The HFR of 1.1 means that during all the movement of the focuser, the “best” position (that is, smallest HFR) was chosen.
If the seeing is poor or there are mechanical issues, the lowest HFR reading might still produce a poorly focused image. You will also find that the AF routine might finalize at an HFR of 1.1 but the next full exposure frame may show an HFR of 1.8. That does not mean the frame drifted out of focus (but it could), its just means after 15 minutes of exposure the star images built up to be larger. Those larger, average star sizes increased the HFR calculation.
Finally, the HFR calculation will be affected by the focal length of your scope and the size of the pixels in your camera since those factor affect the size of the star images. If you are imaging at 2400mm your HFR will always be larger than if imaging at 780mm. In the end, the smallest HFR is the best focus regardless of the actual value of the HFR.
If you shoot binned exposures, the HFR will be about half what you will see if you switch to unbinned. For reference, my scope has a fl of 780mm and my camera has 5.4 micron pixels. On a good night I see well focused images at an HFR of 0.60 when shooting 2x2 binning. This is from the AF routine, not the final sub.
Also, the HFR is in pixels; not in arc seconds.
Charlie