I think it would be useful to have a simple % gauge that can be directly filled by the user.
In projects, it is a common practice to manage a « % completed » value per sub-task and calculate the total % completed in the main task. The administrator should be able to define 3 or 4 preset values associated with a text description, such as « 10% - begun ; 30% ongoing ; 70% closing ; 100% complete ».
A number column can be used as a workaround but this isn’t a very good solution since you can’t force preset values or prevent incoherent values (>100%)