Approval Rule: Approver must be of a certain Grade or above

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I have a requirement for an Individual Compensation Plan where the approver must be of a certain grade or higher. 

In Approval Rules (Manage Individual Compensation) I can't see a way of specifying the grade of the approver. Does anyone know if this is possible and if so how can it be done? I've played around in BPM Worklist and I can't see how to specify the grade of the approver.

Thanks

Russ