Journal approval assignee top participant

Summary: We currently have a journal approval rule (FinGlJournalApproval) where the journal will go to the submitter's line manager for approval, then if it's not actioned within a certain amount of time it is then escalated up another level of the line manager hierarchy.

We wish to restrict how high up the hierarchy the approval will go (i.e. we don't want it to go higher than our heads of departments). I was hoping we could achieve this by referencing the job level (40) in the Top Participant field of the rule, but I've not managed to get this to work. Would anyone be able to shed some light on how this can be achieved?