Guntech is correct: there isn't a pat formula. Try this: put on the heaviest jacket you are likely to wear when using that gun and mount the gun as you would when hunting - in the standing position. The thumb of your trigger hand should be about 1" forward of your nose. Less than that and you risk your nose but much more than an inch will hamper you when shouldering the gun - especially if you have heavy clothes on. That set up should also work if you have to shoot prone, sitting or kneeling. Once the length of pull is correct, adjust the position and focus of your scope for the resulting eye relief and you are good to go.