I did a forum search here on Nitroexpress on how to determine my length of pull and did not find any answers. I expanded my search to some other forums across the web and I wanted to share my results so this information will be available here. Nothing below is mine - this is all cut and paste from answers and posts on various message boards across the web:

How to determine your length of pull with a rifle?

Put your arm on the table, bent at a 90 deg. angle and measure from the top of your bicep to the last joint on your index finger.
There are a variations, but this will get you there.....
This would be a summer time LOP....I would subtract at least 1/2" for a wintertime or year round LOP (allows for a coat)
It is better to have it a little short than to have it to long.

The measurement of the gun would be from the trigger to the recoil pad.

How do I determine length of pull when choosing a custom stock?

The length of pull or LOP is measured from the forward face of the trigger to the end of the butt plate or butt pad. It should be measured to the point halfway between the heel (top) and the toe (bottom) of the butt plate.

LOP is considered a very important measurement for any stock because the length of the buttstock will greatly affect how well you can hold your rifle and how well you will shoot.

If the LOP is too short, you will tend to pull your shots to the right if you are a right-hand shooter. If the LOP is too long, the rifle will tend to ride upward and outward during recoil which will usually make you shoot low and to the left if you are a right-hand shooter. Reverse these directions if you are a left-hand shooter.

Correct LOP can be determined by placing the buttstock along your forearm. Slip your trigger finger onto the trigger and the rest of your fingers around the pistol grip or wrist just like you would do if you were shouldering the rifle. Look down and see if the face of the butt plate or butt pad rests against your biceps.

If it is just touching the surface of your biceps then the LOP is very close to being correct. You can further test for a correct LOP by shouldering the rifle and relaxing your right arm and letting your elbow drop as low as possible without being uncomfortable. Your elbow should be approximately in the centreline of the side of your body.

If it's too far forward the LOP is too long. If it's too far rearward, the LOP is too short. These tests are just initial indicators of correct LOP. Because of variations in shooting styles you may still need to lengthen or shorten the buttstock to gain a correct fit.

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Stephen has given a good definition of length of pull as well as describing the most generic of ways to try to get an idea about length of pull. But neither of those methods will truly establish your real length of pull. And the approximations that they will give can be decidely wrong.

Most of us think that length of pull is just the one measurement and that's that. But there is a long list of variables that get included in just that one measurement.

- How long is your neck, is it shorter or longer than the average?

- How fat or skinny is your cheek on the stock-side?

- How much muscle do you have in the pocket of your shoulder?

- How developed are your pectoral mucles? Your biceps?

- Does your clavical protrude across the top of your shoulder?

That's the short list and you can see how picky it gets real quick if you want a true custom stock.

If you are looking for a true custom stock, I suggest that you go to a professional stockmaker. They can do all of the right measuring and assure you of a perfect correct fit. If you are just getting the length of pull for a fiberglass stock, you should be measured by a professional as well. If you think that you do not meet the "average" description of the "average" shooter, take a look at an adjustable stock in fiberglass.

I'm not trying to beat this into the ground but I am an example of a non-average shooter and I have to make custom wood stocks for myself and because fiberglass doesn't allow for all of the adjustments in a regular stock, I have to use the adjustable ones. It makes a huge difference!

FWIW - measuring from my bicep to the pad of my trigger finger is 13-1/8" and my correctly measured length of pull is 14-1/4".

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Are all important measurements, however, they are more important to cast-on/cast-off and drop.

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One other thing to consider on determining a trigger pull length for hunting rifles, is how much cothing you will be wearing when you are out in the hinterland. Those of us that routinely hunt in -30 or -40, and have to wear parkas and many layers of clothing, will find that a slightly shorter than optimum trigger pull length is required.

On several occasions over the years I have seen southern boys that are use to hunting in T shirts, get the butt of their custom rifle hung up in their arm pits on the way up and miss out on a shot.

--- and last, a link to some more information found:
http://www.peteblakeley.com/custom.htm