Hello,
the workflow percent completed is not maintained as a property on any node, so it can not be used in rules. The fact that you can select it in a rule is based on Alfresco's way of defining task types, which can technically be considered to be regular node types as well. The property in question is only available as variables / properties of a task within a workflow, so either you define a rule condition that is able to extract that information from a workflow, or adapt the workflow in question to do the logic in question.
Regards
Axel