For rifles, aimed shots, length of pull isn't critical so long as the rifle can be mounted without hanging up. For a 9 year old I'd err on the long side else guaranteed it'll be too short next year. Scope eye and nose not clearing his thumb.
Save the pieces. My first gun was a Stevens .22/.410 and Dd saved the cut off piece. Eventually I replaced the cut off piece with a leather spacer (for looks) and it's in the shotgun cabinet. It ain't leaving except to get shot.
The old school method of fitting isn't perfect, for shotguns anyway, but gets you in the ball park for people of average proportions.
