On second thought, I wouldn't actually use statistics at all. I would simply set all defaults to a low value in the single digits percentage range on average over a period of one minute or so.
The overwhelming number of apps need next to no CPU (on average that is. Spikes are OK) during normal operation, and that includes text editors.
Other applications, such as games, need all the juice they can possibly get. Users usually know why that is the case and will be happy to grant them that permission.
There needs to be a way for an app to request a temporary suspension of resource limits, e.g. for running batch job that should finish as quickly as possible.
Each website should be treated as a separate application and needs to compete for resources with the ads it runs. So there would be an incentive for website owners to keep the resource usage of ads in check.
