Shibby, what's your rule of thumb? Two fingers between strong hand thumb (wrapped around wrist of stock) and nose?
No rule. I've heard the ~1" from cheek to thumb rule quoted before.
Try video on a smart phone.
Kay Ohye, one of the greats of trapshooting, says that the stock isn't too short unless you are poking yourself in the nose. Many people shoot a stock which is longer than necessary.
The vast majority of cheek slap problems that I come across are a pitch issue.
As noted, I'm not the one to give direction, but I know for sure that holding any gun with my thumb poking my nose would be very uncomfortable. Or my nose touching my cheek. I do have a long neck.
I've noticed if my gun is slapping, adding a larger pad (from .75 to .9") has solved the issue on two shotguns. Minor change, big improvement. I have an adjustable stock so I can work out some of the other fit issues through adjustment.
I will end this saying I also don't think many people hold their guns correctly (stance) which then makes gun fitment and advice even harder to provide.
Once again, consider it's free advice, but I feel people should square their shoulders more to the target, and put much more weight on the front foot. Similar to a goofy looking pistol stance, it seems to just work. Most people who are 50/50 foot weighted, or worse, weighting their rear foot, I bet have more issues, or different issues with fitment, etc.
I "step" into my gun when shouldering it. I also go further than most with almost all my weight on the front foot, but that's just me and don't advise doing the same.
This:
https://goo.gl/images/yhpNWr
vs this:
https://goo.gl/images/DlXaxZ
Here is another good example:
https://goo.gl/images/mLzIwM