Expected dS/dN ratio for exome
There isn't a fixed value for the human exome AFAIK, nor would it be a good idea to have a fixed value for the exome because different regions and genes can be under different selective pressures.

The norm is to compute the ds/dn ratio and then test against a null hypothesis of the ratio being 1 to infer if your region is under selection or not. If it is greater than 1 it indicates positive selection and if it is less than 1 purifying selection.

You can find a  decent introductory treatment here.