I've been helping in the Great Ask Ubuntu Spam War recently, and one thing I've noticed is that 10-20 flags usually isn't enough during a wave. It feels, quite frankly, terrible to have to sit out during a spam wave.
Therefore, I propose the following: If there is currently a spam wave (i.e. posts flagged out of existence > x) , give users x * n more flags than usual.
The implementation details are arbitrary and up to the SE people, of course. I'm planning on fleshing this out more in the future, but I'm just throwing this out there. Maybe someone has the ultimate formula.