This confused me a little:
It's reduced by 15%
for every super bump you've done in the last 30 days
i.e. the 0.85 multiplier is raised to the power of super_bumps_last_30_days. So if you've done 3 super-bumps your fourth super-bump will be 0.85*0.85*0.85 strong or about 61%.
I've bumped a few threads in a row and each subsequent one is definitely getting a lower score, so it appears to work as described as far as I can tell without seeing the actual numbers.