We use a report that looks at the following filters over a 3 month span:
1. Individual attendance
2. Household Attendance
3. Household giving
4. Small group involvement
5. Ministry team involvement.
That said, a general rule of thumb is that only 1/3 to 1/2 of your adult "regular attenders/members" will show up any given sunday. I'm not sure who said that is a rule, but it holds true for us! We have 1650 adult members/regular attenders, and though our "Sunday attendance" runs 1400-1500, kids make up a huge percentage and several hundred are counted twice (two service hours with Sunday School classes offered during both...many attend a class and go to church or vice versa). So my estimate based on the data I can gather is only 600-800 adults show up each Sunday, which is right on with 1/3-1/2.
We do have "Connection Cards" which we put in the bulletin for people to mark attendance, submit prayer requests, request info, etc and we are able to register a couple hundred in attendance for each service that way. It's not an exact science, but our record keeping has definitely improved. We are still working on the "following up with missing faces" part...We have used the data to move 50+ people from regular attenders to becoming members because of the data though. That's been encouraging!