Bill,
I have completed a spreadsheet for the AT & T Wireless usage of your group.  I'm not sure how much of help this will be to you as of now, since I just got Kate on the group plan and also a couple others (that were set-up before my time).   I can go over it with you if you have time, but it might be something you would want me to track for the next couple of months to see how much it varies.    Just let me know.
Thanks, Jill