There are two different lengths of pull seen on Muzzle loaders, depending on the kind of buttplate that will be on your gun. If you are shooting a Hawken styled gun, with the severe half moon buttplate, it is designed to be mounted out on your upper arm. Generally, such a rifle requires a longer LOP to fit properly. Otherwise, you find yourself laying your face down on top of the comb, and looking down the sights sideways, only to have that gun rise up and smack you in the cheekbone when it goes off. The rifle should be brought to your face, not the face to the rifle to avoid this little surprise.
The other style of buttplate is flatter and is often referred to as the " Shotgun style " buttplate. It is to be mounted in the pocket of your shoulder created by raising your elbow up even iwth the height of your shoulder. This brings the rifle closer to the center of your body, which shortens the LOP needed to fit you well. Mount this kind of stock by lifting the gun up, and bring it back to your should and cheek in one movement, while you face the target with your head up.
With the longer drop at comb of most " shotgun style " buttplate guns, the LOP is often much shorter. I have used an antique shotgun that has much drop at the comb( center of comb to a line extending back from the top of the barrel) and about 3 inches of drop at the heel( top of the buttplate). It had only 12 1/2 inches LOP, but It came right to my line of sight every time. I am over 6 feet tall, and most of my guns, with less drop at comb, are 13 1/2 inches LOP. For a Hawken style gun, I have to have my LOP set at 14 inches, and more to shoot the gun comfortably.
I bought a stock gauge from Brownell's years ago, and have put it to good use over the years. I can guarantee you that I pulled that gauge out when I had that antique shotgun with the 12 1/2 inch LOP to find out how in the heck that gun could possibly fit me, and feel so right!