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: Dry pipeline...it's been a crazy summer...
Recent Meads: Simple Cyser '14
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....