Max power is 364 or so (limited by max activity in a year).
Oh, yes, I looked whole activity and rounded it to 1000.
It will not go down to zero. It's reduced by 15% from the previous value, not by 15% from the original value, so something like this: 100% -> 85% -> 72% etc.
That is base power minus 15% and then modified power minus 15%. This confused me a little:
Once per 24 hours, you can click a topic's bump link. When you do this, a modified bump power is calculated as your base bump power minus 15% for every super bump you've done in the last 30 days, and this is added to the topic's bump score. In other words, your modified bump power is base_power*0.85super_bumps_last_30_days.
I have to see this in combination with
morvillz7z's post (@mod I promise I will delete posts).