A light source like the ceiling light, or pointed towards the window during the day would be fine.
O n my QHY8 I have the gain set at 0 or 1 and it hits 65535 just pointing at my ceiling light.
Not sure if the QHY8L is the same, I am going from notes I made when I borrowed one and had around 14 for the gain, you mileage may vary!
Ann offset of 120 will do you fine, but if you want a more detailed procedure, then run through this one:
http://qhyccd.com/ccdbbs/index.php?P...2671.0;id=2734
Brendan also has the same document linked on his resources page under "QHY9 calibration" but it is valid for any camera.
http://brendanmitchell.net/?page_id=213
Lots of other useful stuff on Brendan's page too.