Using formulas within a filter

Please help! I am charge with doing usage analysis in my company. We have usage stats and make a lot of use of filters. But if I am using a filter, how do I make a formula using only the numbers in a filter. For instance, I used a filter to only show one content type (book discussion guides). But when I try to use the auto sum button to show the average usage of book discussion guides, it makes a formula using all the types, including the ones I filtered out. I know I can use 101 to make a sum formula with only numbers in the filter. Is there a way to do this with averages, as well? I am tired of counting cells.