I am afraid that the Width of the GroupHeaders cannot be set to percentages - actually it depends on a few other things except the GroupHeaderContentTemplateSelector, such as the count of the intersecting appointments inside certain groups, and is calculated in the internal code of the ScheduleView. In the GroupHeaderContentTemplateSelector the set Width will be considered as a MinWidth.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.