The easiest way to do this would be to upgrade to v1.9.x - from what I understand about your requirements the (significant) Gradebook revisions in 1.9 will do everything you want to do with little in the way of effort. Save you having to re-invent the wheel.
Hope this helps,
NetSpot Pty Ltd
Australian Moodle Partner
New scale (based on 5) on Grade Book.
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