Here is a link to the sub-exposure calculator:
http://www.ccdware.com/resources/subexposure.cfm
For some background on factors that may impact your choice of sub-exposure parameters, here are some links, in increasing order of complexity (sorry!). The first one should get you started:
http://www.hiddenloft.darkhorizons.org/notes/ShortExpAppNote.pdf

The next one is the mathematical basis of the calculator for signal to noise considerations:
http://www.hiddenloft.darkhorizons.org/notes/SubExposures.pdf

And this one includes the impact of dark subtraction:
http://www.hiddenloft.darkhorizons.org/notes/DarkSubExp.pdf