In my experience, pitch both affects the perceived recoil as well as the point of impact .. the former much more so than the latter. Too much pitch down will normally cause the gun to come up into the cheek and it will also tend to shoot just a tad lower. Conversely, too little pitch or in an extreme case negative pitch [pitch 'up'] will cause the gun to want to slip from the shoulder pocket and will tend to have a slightly higher point of impact.
I tend to think that the more punch in the face with excessive down pitch comes into play more with a premounted gun than not and that a reverse or negative pitched gun was set up to shoot vertical overhead targets. That has been my conclusion in any case. In the former instance, I have had several different guns [normally O/U's, but I'm not sure it makes that much dif as to type] that were bruisers and reducing the pitch eliminated the issue completely.
The only gun that I have personally owned with negative pitch is a light weight French made SxS that actually works fairly well on close in very high tower birds, but it honestly would be much better, IMO, w/a slight amount of down pitch. At present it remains unmodified w/original horn butt plate and as such is not usable for any other type of shot, that I can determine. I would almost dare someone to premount it and keep the butt in the pocket when fired, I cannot.
My thot is that if you have a vertical shoulder pocket, then zero or neutral pitch may be called for. However, most people will have some amount of build 'out' when they are shouldering a shotgun; big & heavy guys and well endowed women could be expected to have more build out than thin guys and gals. Said another way, thin people would need a bit less pitch than heavy people. Both of those statements are generic and there are certainly exceptions.
In terms of easy measurement .. using a door jamb and checking with a square that said jamb is square to the floor .. with the butt firm on the floor & the rib against the jamb a space of somewhere from 1.25"~2.0" measured at a height of 28" up the jamb would be about right, for most people. I find 1.5" at that height is near perfect for me. It can be stated in degrees if you like, but most of us find a tape measure or ruller and door jamb handier to take a quick measurement with. A table top & square as previously described works fine for layout before changing it.
I think that the shooting styles of earlier times were quite dif from today's and as such very few people placed their cheeks firmly to the comb. Rather, they tended to bring the bbls up to the target with their heads erect and therein lies the reason for the drop we find in many older guns and also, in some cases the excessive pitch down, if present, may have been to better accommodate the heavy and thick clothing often worn, but that is a guess on my part about the pitch.
I have also noticed that guns purpose built for International skeet tend to have more pitch down than guns built for American skeet.
