Configure a certain “Done Ratio” for some Issue statuses, while keeping the ability to manually set it.
The plugin’s global settings (Administration / Plugins) allow to specify a fixed % Done value for the desired issue statuses. Statuses where no ratio is specified will not change the issue field when the issue enters this status.
The issue % Done field is editable all the time (as long as the relevant Redmine setting Calculate the issue done ratio with is set to Use the issue field in Administration / _Settings / Issue tracking).
I wrote a tutorial style article illustrating the creation of this plugin.