[avandonk]

I am working on a top secret method where you take flat frames with a ground glass screen in front of the optical train only illuminated by a dark sky and/or moon for the same exposure conditions as the light frames. That is same long exposure say six minutes. This means any fluctuation due to noise is also taken care of in the flat calibration stage. I always use in camera noise reduction and in conjunction with this would be even better.
Below is a 100% crop of a six minute flat with a tissue (not as good as ground glass screen) over a Canon 85mm F 1.8 at F2.8 with in camera noise reduction on. This shows the 'holes' that noise reduction produces.
You can work out the rest!

Bert