Adding to Ted's comments, it is important to understand that the purpose of the lead block is to manage the tension on the headsail's leech and foot. It works similarly to how the mainsheet and outhaul affect the mainsail's shape. Move the car forward and the sheet's tension increases on the headsail's leech while it reduces tension on the foot of the sail. A rounder shape with a closed leech.... powering up the headsail. Move the car aft and the reverse occurs, more tension on the foot creates a flatter lower portion, just like the main's outhaulwould, while the lessening tension on the leech opens the top portion, effectively flattening it.......de-power the headsail. So.....You want to bisect the sail with the sheet to get equal tension on both edges. This is the neutral position. The track's midpoint would be at this neutral position. As Ted explained, extending a line from the mid point of the luff down through the clew to the deck will guide the attached sheet to the block's neutral location. Simply put you are bisecting the corner angle between the sail's leech and foot. If the sail is hoisted, you can lay a simple protractor over the clew, use a felt pen to indicate the bisecting line, then use the attached sheet or something straight to indicate the neutral point on deck. This would also be the mid point of your track, and I would try to get at least two or three track adjustment holes ahead and behind the mid point .. If you use more than one size headsail, or roller reef your single sail often, you should mark their neutral positions on you deck so you can allow for a longer track if needed.
Okay, I hope that made sense, because I want to add one other consideration. That is the track's lateral (athwartship, side to side) location on the deck. This will determine the all important tack angle....which is the angle created by a line from the stem to the jib lead block and the centerline of the boat. The tack angle is often determined by the characteristics of the sail and the boat it's designed for.... so it would make sense to get the sailmaker's input, and also that of the boat designer. Boats that are designed with racing in mind will readily have this data... so it's worth a try to reach out to an expert. Some boats will have very little optional space on the side decks and you will actually see tracks on the cabin top... a Sonar, for instance, which s a popular one design racer has fixed leads mounted on the cuddy cabin roof. On my Catalina 27 I have a long track on the toe rail that literally runs from the foredeck to the transom... often called a Genoa track...it will accommodate any size sail and I have 3 sets of sliding cars, plus the Garhaurer EZ glide lead system... (allows me to adjust the car position under load).... And..... I also have a short 5 position track mounted inside the genoa track that gives me a tack angle option for the genoa, And I have rigged a "Barger Hauler" for the smaller headsails that won't fit the inside track.
Confused? .... that's why you want to understand what the lead block is and how it works. At this point the sail determines its location, but later, if you add some more sails.... their design will incorporate the peculiar aspects of your boat and how you want it to perform.
Good luck.... sorry for the long winded post....I can get carried away sometimes. Before you start drilling new holes, you might cut out some cardboard replicas, place them around the boat to depict your options.... with the sail hoisted of course... maybe even take a picture or two to show us... oops ... at it again
