Pitch should be set at an angle that gives the most butt surface area contact with the muscles of your shoulder notch. This will change when you shoot down, horizontal or up & therefore cannot fit perfectly.
The best pitch angle for you is a feel thing from shooting the gun & how it feels & best suits you.
There is no standard measurement & guns for different purposes will have differing pitch. e.g. A live pigeon gun set up for rising & high targets may have a neutral to up pitch while a field gun may be more neutral to down.
Put the guns butt flat on the floor & the barrels against the wall.
If the barrels go straight up the wall then pitch is neutral. Muzzle end of barrels come away from wall while chambers touch wall is down pitch.
Muzzle end of barrels touch the wall & the chambers come away from the wall is up pitch.
Down pitch is more common & generally in the area of 1" to 2 1/2" down on factory guns. Measured from muzzles to wall. Note that length of barrels accentuates the pitch measurement. On 26" barrel set the measurement will be less than a 30" but the angle is actually the same. So another way to measure angle is with the barrels laying rib down on a table & then use a protractor to measure the butt angle. Compare this to another of your guns that suits you in its pitch angle.
Hope this helps.
O.M
