I've used shirt boards & plastic cards for shims when necessary for a bit of tweak. However, somewhere between 1.5" & 2.5" pitch down at 28" from the floor [door jamb method] is generally going to be the range you will find most useful & comfortable, predicated on how you are configured.
Now for the fun part, opening the Pandora's box of pitch. Lots of pitch down tends to make one shoot lower and insufficient pitch down or even reverse pitch [pitch up] tends to make one shoot higher. There are those who will argue against this until they are blue in the face, but I find it to be an accurate & valid statement from my own fooling around, aka testing.
Guns to be used in the field, as in behind dogs or just impromtu shooting will require a bit more pitch down than a pre-mounted target gun .. if you expect to shoot well.
International Skeet guns tend to be purposely stocked w/ a LOT of pitch down.
Trap or DTL ['down the line' in England] guns tend to have less pitch down.
Our own version of Sporting Clays was quickly prostituted with the 'any mount' [say 'pre-mounted'] rule, so that many Clays guns are not field guns at all.
What you find comfortable for a dedicated clays gun plus .5" pitch down at 28" from the floor should prove very close to what you will shoot best in the field.
Just my thots.
Kind regards,
tw
