Thank you Ray - your method is certainly easy to apply and understand
Rick- I have just measured mean value for a central area in several bias frames collected over 45 minutes. The greatest difference was 1.2 ADU, which I estimate to be around 0.3e. Would it be worthwhile using overscan in my case? I managed to find a way to turn overscan on and off, and it gives access to about 150 additional columns of pixels on the right-hand side of the image.