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: Simple Cyser '14 (Nelson Sauvin Dry Hopped!)
Primary: Simple Cyser '15, Basic Spiced Cider, Spur of the Moment Graff
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider
Planned: Some sort of SMaSH. Otherwise, so many ideas, not enough time to brew them....