How do I substitute stevia for sorbitol?
According to stevia.net one tablespoon of stevia (in powder or liquid form) is the 'equivalent' of one cup of sugar.

Similarly, lowcarbdiets.about.com tells us that sorbitol is 60% of the 'sweetness' of sugar.

Using sugar as a common denominator, 1/2 cup of sorbitol = .83 cup of sugar and so you should use ~.83 Tablespoons of stevia to gain the relative sweetness. 


  1 cup of sorbitol = 1.66 cups of sugar 'sweetness', so .83 cups of
  sugar would be 'as sweet' as 1/2 cup sorbitol


That calculated, I would mix 1 tablespoon of stevia with 1 cup of unsweetened applesauce and then use 3/4 of a cup + 1 tablespoons of the mixture, and then adjust from there. 


  1 cup = 16 tablespoons, .83 cup = 13.3 tablespoons


Conversions like this are admittedly 'imprecise' but this should give you a good approximation to start from...your mileage may vary.