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IfI you are shooting bias frames which are a really short exposure to show the noise from the readout process then there is no need for a dark for the flat. The dark is the bias frame. Take 3 and median combine them. Take 3 flats so you get about 1/3rd of the saturated exposure of your chip. Median combine them and subtract the master bias. That is your flat. No need to use the bias anymore after that you are done with them.
The dark then is 6 or more exposures of the same length as your light and ideally at the same temperature although adaptive darks in Images Plus reduces that need somewhat.
These are combined using sigma reject. That is your master and it is smart practive to make them a normally used exposure length that you can use over and over say 5 or 10 minutes for a DSLR. Have a master dark for each used common exposure time. Make a little library.
The darks will remain valid for a while but chips degrade over time due to radiation damage from cosmic rays.
So redo them from time to time and in your case as it is not cooled from season to season to best match the temperatures (typical for a CCD is thermal noise doubles every 6C or so).
To learn photoshop try Wodaski's Zone System book. I think it has everything you need just about.
Greg.
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