Estimating gain of my camera

I wanted to try out using the subframeselector script to assign weights to all frames. For that, I needed the gain of my camera. I can't find the data sheet from FLI anymore, but Richard told me a quick way to estimate the gain good enough:

1. Take two flats.
2. Raise the level of one of the flats slightly.
   I am doing that in Pixinsight by using the Pixelmath expression $T[0]+0.2.
3. Subtract the other flat from this first flat
   Again, in Pixelmath: Ha_01270-Ha_01269. (these are the names of the flat fit files).
4. Measure the standard deviation in the middle.
   In Pixinsight, I create a preview in the middle of the new image and with the Statistics process measure 5.1259e-003 = 328.06 DNA (*64000)
5. Divide by sqrt(2) = 231.97 - this is the noise (or to be exact the quadrature sum of shot noise and read noise)
6. Now subtract a bias frame from one of the two flats
7. Use the same region (I drag and drop the preview that I created in step #4) and measure the average value: 6.1413e-001 = 39304.32 DNA (this is the signal)
8. Now, gain = signal / (noise^2)
   gain = 39304.32 / 231.97^2 = 0.7304