I avoided this thread because I hate to disagree with other shooters but now it's time to write something.
The correct pitch for a given shooter depends on several factors: where on the shoulder the gun is mounted, the stance used by the shooter, the body posture of the shooter when he or she shoots and the chest configuration of the shooter.
The pitch is correct when, as the gun is being mounted, with the barrel raised to a normal shooting height, the whole recoil pad, top to bottom, makes simultaneous contact with the shoulder.
I gave up calling pitch positive and negative following a phone discussion with Dennis DeVault. He describes what I would call "positive" pitch, negative pitch. I have forgotten his reasoning but being Dennis, I accepted that I may be mistaken in my definition of pitch.
Pitch described in inches invites another problem - the length of the barrel or the point on the barrel from which the distance to a vertical surface is measured.
Describing pitch in degrees the way the English do is the best way to describe pitch but few have the ability to accurately measure pitch in degrees, especially when the bore axis is used as the basis rather than the rib, which can slope toward the muzzle.
The only value of measuring pitch is to allow a basis for comparison so the pitch can be duplicated (or corrected) on another stock.
Too much or too little pitch can cause problems. When the toe of a recoil pad or stock protrudes too far, barrel rise will increase and the gun is more difficult to mount consistently. When the condition becomes more extreme, cheek slap occurs. Pain where the toe of the pad makes contact with the shoulder is also a distinct possibility, especially for women.
When the top "heel" of the recoil pad protrudes too far, the gun can slide up on layers of clothing during recoil and can also punish the shooter's cheek.
Pad adjusters are sometimes used to compensate for the wrong pitch. When the toe of the recoil pad sticks out too far, the whole pad can be rotated with a pad adjuster to put the toe of the pad in the armpit. Without the ability to rotate the pad it can even cause gun canting.
You may be wondering how the stance used by a shooter can affect the correct pitch. If the shooter shoots a shotgun like a rifle, with the shoulders nearly aligned with the direction of the shot, the recoil pad contacts the shoulder differently than when the shooter faces targets more directly.
With the body rotated too far, the toe of the recoil pad must protrude farther to make contact with the recess of the armpit - to spread the recoil over as large an area as possible.
On LOP comparisons: These can be of limited use. This is because the height of the gun mount and the length of the shooter's neck play a significant part in the correct LOP for shooters. The "correct" length is usually described and the distance between the tip of the shooter's nose and the thumb - 1" to 1.5".
A long neck and/or a low gun mount both require the shooter to lean his or her neck forward to put the cheek on the comb. This lean requires additional stock length because necks have length.
The stance used by the shooter also significantly affects the LOP. The farther the shooter rotates the stance, the longer the stock must be.
Even the type of shooting can affect the preferred LOP. Shooters of low gun disciplines like FITASC and hunting often prefer guns with a shorter LOP to facilitate quick gun mounts.
Rollin