I bought a light box from Exfso also. Great bloke, great light boxes.
I used to have a QHY8, so taking flats was easy by just plonking it on the top of the scope and setting the exposures to around 0.07sec and then taking 50 of those.
With the QHY9M and filters that I have now, it has a minimum shutter speed of around 0.5 sec, otherwise the shutter can be seen in the flats if the exposure is shorter. To alleviate this, I installed a pot I had laying around and dimmed it down to very low and take the flats over 2 - 3 seconds.
The results are the same, just the brightness and the exposure length is different.
For me, the light box has always worked the best, the results are very repeatable and you don't have to deal with changing light conditions when trying to take sky flats. There is software out there that can help with sky flats by adjusting the exposures to achieve the correct ADU count per sub frame.
You want to acheive a nice flat frame of up to half the ADU count of your camera, but you will need to experiment with this. The QHY8 produced the best flats at around 10,000 -11,000 and the QHY9M gives better flats at 20,000 - 30,000, with my set up.
There is an article here on the site about flats if you haven't read it.
http://www.iceinspace.com.au/63-211-0-0-1-0.html
Flats really can be one of those mystery things with astro photography, but once you "get it", you'll wonder what all the fuss was about. You also need to find out what works for you.
