There may be something here...
http://www.pbpgroup.com/modules/wfse...hp?articleid=7
that will help you with the averaging.

Darrel's premis is that if you want to average several items together then do the math in a sort of "reverse" fashion. ie. instead of adding 3 values together then dividing by 3. first divide each value by 3 then add them together. This will help of avoid overflow situations.

He does a much better job of explaining though.