I just found this neat billable hour calulator at: http://www.envoyglobal.net/jdbliss/test/calculator2.htm

Check it out. You input your commute time, your work hours, your non-billable distractions everyday and it spits out results that show how much billable time you're going to have to do at home in order to acheive your minimums. Pretty cool -- not something that you couldn't do with a calculator, but very easy to use and handy.