the formula you have will determine the average for the report...as there is no group qualifier.
with that said, there are at least 2 ways to skin the cat that come to mind.
1) place you formula in a footer...since a footer is only displayed once, the average will be as well.
2) check the option to supress if duplicated (Format object, Common tab)
Personally, I would go with option 1, again, since it would display the total for the report, I would place it in the report footer...but that's just me.
HTH