Ahhh, I understand, Leon.
What the DSI tutorials suggest to do (and you may want to try it) is to have a Library of Darks.
I tried it and it does help reduce time. I took a stack of 30 second darks at temp 2 degrees, a stack of darks at 40 seconds at 2 degrees etc. Then the next night was zero degrees so I did the same stacked 30, 40, 60 second exposures at zero degrees. I now have a library of stacked darks at different exposure rates and temps. What I do then is, if it's say '20second lights on an 8 degree night' I take my lights and get the darks from my library by selecting the closest (something like '20 sec @ 7 deg). It's a lot of work, but worth it for the clear nights.
What you could do on cloudy nights is build up a library of Darks at different exposure times and temps. Good use of those cruddy nights
