By thumbhole do you mean the position of the nose of the comb? If so I locate this by 1) the drop at comb and 2) the distance from the front trigger on a double trigger gun or the distance from the middle of a single trigger to the nose of the comb. Measure a gun that works for you and compare it to the one that smacks your finger.
Jeff

"We are men of action. Lies do not become us."
Wesley