I have update Roastime v.2.2.0 to 2.3.1 and found the overlay mode worked differently from previous version.
It seems it adjust power and fan automatically to match the old curve which I like it.
But I did expect this new feature. And it made me wonder if there is any documentation for the new features in new software release I could get.
I did 3 roasting smoothly with new software and new firmware calibrated fan. New software is so good.