I direct a community band, though we're only two years old so our process is constantly in flux. We don't take roll, but we try to roughly keep track of absences. I would say that 80% might be a little hard, but a good number to shoot for. On the other hand, several of our sections don't have anyone waiting in the wings, so I'm not sure what I would do. (As we speak, I have just given our bass trombonist permission to be late every week rather than lose him entirely.)
What we do is constantly exhort folks to show up, plus push hard for same-day notification of absence (I think that reduces skipping, as people don't want to tell you they aren't coming). It has worked to the point that we always have functioning rehearsals and I think we pretty much hit the miss-one-a-month target I set.
If you come up with a foolproof approach, please don't hesitate to let me know!
If life is like a box of chocolates, my opinions are like a box of Vandorens: you may have to go through the whole box to find a good one...