The old crook of elbow to finger ia a real common recommendation but it is only a very rough approximation. It takes into consideration arm length but does not account for length of neck, thickness of neck, shoulder construction etc.
Take a gun you have now, put it up to your shoulder with your eyes closed and settle at a comfortable spot on the cheek rest when you mount it to your shoulder while standing. Open your eyes but do not adjust your head, have someone measure from the front of your nose to the first knuckle of the thumb on the pistol grip. Repeat this a few times, writing down all measurements. Now measure the LOP of the rifle, being the distance from the but pad to the trigger. The average of the measurements you took should work out to be 2"
I found this from Boyds. Very similar method but perhaps a little easier. One thing to watch, when he talks about the centre of the butt pad and centre of the trigger, he means the up and down centre, not the middle of the pad. You measure to the end of the pad and the front of the trigger shoe.
https://www.youtube.com/watch?v=qOgIPbzKHWI