The starting point is 2" longer than the distance where your arrow contacts the rest. However, you can go as low as .5" with some rests to achieve proper spine. Don't use your DL as the starting point. Depending on your rest you can cut some arrows 3" or 4" shorter than your DL.
Unless all the manufacturers you are using provide the exact same spine you often times have to change the arrow length to get a proper spine.