7mm rem mag head space.

[HuntinGuy]

I am new to reloading and i am attempting to set my head space on my once fired cases. I am aware that belted magnums head space off the belt, but should not the cases stretch when fired and head space off the shoulder? I want to just bump the shoulder back. I tried the Sharpie method and show no marks on the shoulder, also i pulled the pin and spring on my bolt and tried to feel for resistance on my bolt but felt nothing. Should i just bump the shoulders 2-3 thousandths from where they are not knowing how far they sit from the shoulder in the barrel?