you need to do the averaging yourself. You will need to sum up your daily sums and then divide by the number of days.
You can accumulate the sums using running totals (DBlank's forte) or you can use a series of formulas that does the same thing. If your daily sums use some logic to include/exclude values then you can increment your variable in that logic. if it multiplies different columns together to get the sum try something like Average({table.field1}*{table.field2}, {grouping})...if your group is on days, it should work.
as you might have guessed, there are many ways to accomplish your goal, and depending on how your data / report is structured will point to the solution that best fits your situation.
HTH