If anyone hasn't read these they might be of interest
A qoute from part 1.
"if we want a very good estimate of the average dark current (so we can subtract this expecetd valu of the dark current from our lights) we need even more dark frames to average togethor"
futhermore
"How many dark frames should I use ? depends on your dark current. If you have a lot of it and you image gets considerably brighter and noisier as you take longer dark frames you are going to need more darks. If you don't collect enough darks you are going to inject noise into light frames when you pre process the images"
refer this link for articles
http://www.stark-labs.com/craig/articles/articles.html
under 2009