In order to determine the minimum exposure, we have to measure the sigma (std dev.?) on an offset and on a light : Sigma light>= 3.5/4 sigma offset.
No problem with the offset and statistic tab. For the light we would be able to draw an area on the view image, without stars, and get the statistic result.
The rule to fix the MINIMAL time exposure is:
Std_dev of a light background must be at least 3.5 time the std_dev of a biais.
So no problem for the biais, we have to measure the std_dev of an area without star on a light.
This could be done on the view image.
The exposure time is increased until to reach the 3.5 value.
Today, we have to get a light capture and do this measure into a software like Siril or Pixinsight.
1. Capture bias frame.
2. Measure std deviation of whole frame.
3. Take light frame
4. Probe 100x100 pixels area and measure std deviation.
5. Repeat #4 until std_dev is 3.5x bias frame value
Yes the area must be without start.
We have just be able to zoom into the image and draw a right box.
I think you are going to automatize this function. It is not mandatory on my thinking.
Juste a possibility to draw a box into the image and obtain its statistic would be great.