Most Capri sailors like to move the jib car as far forward as possible and leave it there. Moving the jib car generally changes the shape of the jib.
We can change the jib shape three ways:
1. tighten the jib sheet
2. move the jib car
3. tighten the forestay tension and/or the jib halyard using the block system most of us have on our jib halyard. If we had a backstay, that adjusts the forestay tension too.
The jib sheet: In moderate winds, a rule of thumb is to have the maximum sail draft (deepest part of the curve in the sail) about 45% back from the sail's luff, the front edge. You can also ease the jib, then trim it in, stopping just as it ceases luffing. If the jib is too tight, then it will cup and depower the sail.
The jib car: Move the car aft to put more tension on the foot (bottom edge) of the sail as the jib's sheet is tightened. This will flatten the foot or bottom of the sail and twist the top of the sail, thus, depowering the jib. Moving the car forward will place more tension on the top of the leech as the jib's sheet is tightened. This increases the draft of the jib and untwists the sail. This will power up the jib. If the jib luffs at the top of the leech first, the fair lead is to far aft. If the jib luffs at the bottom of the leech first, the car is too far forward.
It is easy to see how moving the car changes jib shape: think if the car were so far forward that it was directly below the cringle (hole where the jib sheet attaches). Then tightening the sheet would pull mostly down on the clew of the sail, thus pulling the leech downward.
If the car were far aft, then tightening the sheet would pull back on the clew of the sail, thus stretching the foot of the sail.
The forestay tension/jib halyard tension: The luff (front edge) of the jib is attached to the halyard on our boats, so increasing tension straightens the front of the jib. Note that tightening the jib halyard pulls the mast forward, so you may need to adjust the mainsheet and boom vang.
Have you ever tried to tighten the windward sheet to shape the jib? We found that if you are beating and want to get it trimmed in as much as possible you can crank the windward sheet and pull the jib closer to the center. It seems to work and combined with the tied traveler you can really beat it close.
The margin of good performance is of course finding that critical angle of attack where you are not stalled, not luffing, and making maximum speed toward the next mark. There are volumes written about it. Certainly a difference of 2 or 3 degrees is significant. If you can use the windward jib sheet to reduce the jib's angle of attack and point higher without back-winding the main you have probably found the best possible setting. As I am sure you know however, wind strength and sea state change and no one position is ideal under all conditions. In most one-design boats I have raced the experts didn't find using the windward sheet an advantage because the boat and sail designers had considered set the angle of attack properly. (on large boats they do all sorts of things and have various devices for playing with the angle, and sometimes physical limitations because of the beam of the boat etc. ).