Brett, take a few dark shots at your desired gain and offset 50 (default) and note the median value. You can get this value by loading the dark frame into almost any imaging software, including SGP, PixInsight and FITS liberator (or many others).
Ray (Shiraz) calculated a bunch of tables for target ADU values while imaging. I’ll see if I can dig the thread up, but it makes everything easy.
https://www.iceinspace.com.au/forum/...d.php?t=155908
IIRC, for gain 139 add about 400 to the median value obtained above. This is your target ADU. Now aim the scope high up at an uninteresting patch of sky on a moon-free night. Run test exposures at a spread of exposure times until the median value in the test shot approximates your target ADU. Job done!
This technique should be repeated for different filters…for example, narrowband filters will cut out more light and it will take longer to hit the target.