I think the ideal is about a foot behind the persons feet. Be careful about hitting the persons feet in front of you, you may end up with a kick in the face. 
If you feel the turbulance, you are usually doing it right. The turbulance you feel is water that is already moving... When you takea stroke, your arm goes the same direction as that already moving water and you end up using less energy AND going faster.
Hi dnavarro!
I hope you arn't practing on drafting during a race!
However, on the other hand, when training with a partner, drafting is a fun way to practice swimming fast with minimual effort! I was a competitive swimmer for 15 years, and in college we had many workouts that dealt with drafting.
The best position to draft off someone is to be off to their side (you choose what's more comfortable for you, right or left) and you want to try and be right up on their hip. So when the lead swimmer is swimming your extended arm stroke reach towards their hip. I've found that this is the best way to feel the draft.
When swimming in college, we would do a set of 10 x 50 yrd swims, where the leader would lead all 10 wearing fins while the drafter would swim besides them trying to keep up with the leader wearing fins and try to swim at a fast pace without expending a lot of energy. You will know that you completed a correct fast 50 of drafting when you feel like you didn't have to try all that hard. Then you can go ahead and switch, so the drafter becomes the leader with the fins. The leader gets the good workout while the drafter learns to swim fast without expending a lot of energy, then you switch it up. It's a fun little workout. Practice makes perfect. It's a trial an error sort of training. Have fun and happy swimming.
PS-I'll know its you if I have a drafter on my hip the next race! 
Wait, what!? It's illegal to draft someone in a tri? WOW...good thing I found this out now, I was about to hold on to someones ankles and let them pull me but seriously, I didn't know that...phew. Thank you.
See you at the races LadyTri, haha. I'm actually doing the Chicago Tri in a few weeks.
I'd say the more confident you are in your swimming ability (especially pack swimming ability), the further up you start. Smack in the middle might be a little scary. I always position myself towards the outside edges and then funnel in. Also, stay relaxed and accept that other swimmers might accidentally hit/scratch/punch. 
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