As with anything else like this, I think you're best off making a solution of stevia with a known concentration, then make a few couple ounce glasses with varying amounts of your sweetening solution. This should give you a handle on where you like the sweetness level, then you can scale it up to the entire batch's volume.
Packaged: Red X Weimea SMaSH
Recent Meads: Mead Day '11 Ginger Metheglin, Mead Day '12 Traditional (orange blossom) Mead, Simple Cyser '12
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider, Simple Cyser
Planned: Hop Metheglin #3 (NZ hops), Trad. Gesho T'ej