Normally, a shooter cannot determine his own Cast Off. Someone else has to look down the barrel of YOUR EMPTY GUN while you mount it normally to your face.
I found it works best if you let the shooter mount the gun to his face, a couple of times, then ask him to do it with his eyes closed. I ask the shooter to hold that position, without opening his eyes, and then walk forward, on the other side of the gun to use a ruler to give me an idea how far off the center of the barrel his pupil is located when he opens his eye. I take a measurement 3-5 times and average the distances, to compensate for my own errors, if any in taking the measurement. A lot depends on the shape of the shooter's face, and how long his neck is, as it affects where on the comb he places his cheek.
A Try Stock would be the preferred way to measure the cast off- but they are hugely expensive, to make- as I have never found a source for them in order to buy one. When you do find a Try-stock, they are almost exclusively made for break open style shotguns, rather than single shots, semi-autos, or pumps shotguns, or even lever action guns. With a try stock that fits the action, you can take the shooter out to a practice field and have him shoot patterns, or shoot a thrown clay targets to see how the new dimensions feel, and watch how he actually mounts the gun to his face when shooting. Its at the patterning boards that the final adjustments in length of Cast Off, LOP, and drop at comb, or at heel are made with the Try-stock. [A barrel- chested shooter will need more Pitch to his stock, than someone who is tall and thin.] Then those dimensions are transferred to a stock blank and the stock is made for the customer.
You don't see many rifle stocks made with Cast Off( or Cast On for Lefties).
I think most smoothbore shooters would shoot a BP shotgun of any style better if the stock fitted them better. So, if you can find someone with a Try-stock to measure you cast off, using a cartridge, break-open shotgun, go ahead and have them take your measurements. They will be the same with most styles of MLing shotguns. Once you know your amount of Cast off, or cast on, you can make any rifle stock with those same dimensions, and they should fit you like a glove, PROVIDED they are fitted with the flatter, shotgun style buttplates, rather than the half-moon shaped butt plated found on some Mountain rifles. THE HALF MOON BUTT PLATES ARE DESIGNED TO HAVE THE GUN SHOT OFF THE BICEP, NOT YOUR SHOULDER. :hatsoff: