The best advice given to me for finding initial car position is:
1. Fold the sail in half along the leech
2. Put a small felt mark there
3. Use a straight edge (tight piece of cord etc) from the felt mark through the centre of the leech to the clew where the jib sheets attach to the jib. On all my jibs I have drawn about a 2 ft felt line through the clew.
4. Adjust your jib car so your jib sheet forms a straight line with the felt line on the sail. This will result in evenly pulling on both the foot of the jib and the leech of the jib. Remember both the leech and the foot are curved!
5. If the wind is not strong enough, you want to close the leech and capture the wind in the jib. Moving the car forward pulls down on the leech, shortening the curve and closing the jib up. This also releases pressure on the foot making the foot more curved (giving the jib more belly) and thus powering the sail up
If the wind is too strong, you want to open the leech to get rid of the wind. Moving the jib car aft puts less pressure on the leech making it more curved, opening it up and allowing excess wind to escape. Also the sheet now pulls more on the foot of the jib flattening the foot curve and and the sail overall.
6. Each of these changes alters the pressure along the leech and the foot. The pressure may be altered sufficiently to cause either the leech or the foot to flutter. This fluttering has little effect on boat speed, its just very annoying. After each alteration of jib car position I alter the leech and luff cord accordingly to take the flap out of the sail. My leech and luff cord are attached to Velcro ends allowing for easy adjustment.
Hope this helps