Ray,
I took a 5-second dark and a 10-second dark last night, and, my background came out to very close to 780 in either image.
Plugging those values in with a gain of 0.81e- ADU, gives a target of ~2870.
I slewed over to Antares and did a 600-second exposure which yielded a background of 8,200-something. Clearly, way, way too long! The sky was quite dark last night before the Moon came up and Antares was at an altitude of about 47 degrees.
By the way, I'm getting the background ADU by simply applying the Range screen stretch (MaxIm DL), which shows the minimum and maximum values in the image. I hope this is the right way of doing it. I don't trust myself with clicking on various areas in the image and then taking an average reading.
The other way of seeing this value is also using the Information window and using the Area option (which defaults to the full frame) and gives a bunch of information about the image.
If it's clear tonight, I'll give it another go and try to fine tune the exposure.
H