How much spring back in length you'll get actually depends on the pressures you are loading to and the condition of the chamber. It can also be affected by how well your bolt and receiver lugs mate. Keep in mind that what stretches longitudinally in a case the most is the short length of brass that forms the pressure ring, which is not long enough to have as much spring back as you might think.
If you want a more accurate measurement from a case, you'll need to neck size and re-fire the same case several times, measuring it each time to see what limit it's heading toward, or continuing to shoot it and measure it and stopping when you start to feel light resistance when chambering, then using the previous measurement. But just bumping the shoulder back from as-fired size by 0.001" to 0.002", as Bart described, is a method that has been used by a lot of people for a long time with good success.
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