[Phil]

I use what they call a light box made on the cheep but woks great. What you are looking for in a Flat frame is a shot of a white background with the same setting as your light frame. Exposure time is work out by measuring the ADU of the ex poser. Eg. My STL11000M has a ADU of 60000 counts. So for this camera i am looking for a exposure of about 30000 ADU. Its about 1 third of your camera tolal ADU. To measure the ADU of the exposure is easy in software like Maxlm DL you just put your mouse on the image and on the bottom of the screen is your ADU read out. Hope this helps there is a photo of my light box.
Phil