But back to the question as answered by BAR. The short answer was this. Arm outstretched, bent at 90 degrees at the elbow, rifle in the crook of the elbow, muzzle upwards, butt in the crook of the elbow. The trigger finger, at the first joint of the finger should be able to fit round and squeeze the trigger.

If not, the answer was to change the butt spacer on site. But in real life, you told the bloke to find a rifle that DID fit. End of!