This is a situation where your SS has to really shine, and show WHY she's at SS. She cannot cheat towards the bag, or she'll get burned on a hard shot to her right - now you have runners on 1st & 3rd, with no outs to show for it. I've seen many game lost at the HS level like that.
The throw by the pitcher will vary slightly depending on how quick the SS is, how hard the ball was hit, how hard the pitcher throws, etc. So there is no exact science, because there's too many variables. But I agree that it almost has to be nearly over the bag or just slightly left. Another variable is as the level of play increases, so does speed. So with a 12u team executing this, the throw may be much further left of the bag, but still make the play. With accurate throws, DPs are more likely at younger levels than college.
In every case, SS has to play her position, use excellent footwork to the bag, make the catch while tagging the bag - all while avoiding the advancing runner AND looking for a potential DP at 1st. A SS that can make that play on a consistent basis is worth every penny of her scholarship!