Bell curve as well. They are so specific in nature, (same spot, same feelings) you will.... (given a couple of cycles) notice ones at the beginning of a cluster that don't even qualify as "headaches". After a couple of those you know... *sigh* a cluster has begun. This is when I usually warn employers, and family. To explain why I might "disappear for an hour or two" until a headache passes.
It then ramps up, and then eventually ramps down. The worst is when you are mid cycle. You keep asking, "have I past the peak? or am I still ramping up?"
It goes without saying, that as predictable as the headaches become, the length and severity of each cluster is something I have only been able to VERY roughly estimate.