I have no idea how you can check a rifle's headspace with a straight-edge.
: Case web separation (3/8" above the rim) is caused by excessive headspace, with only one other possibilty.

: One way to get around an excessive headspace problem is to seat bullets out to contact the chamber's throat, thereby holding the case hard against the breech face when locked up. This usually works or helps in a bad situation.

: .005" is not much variance in headspace. Belted magnums can run up to .012" difference in headspace, from the same box of brass. This is just another reason why some belted magunms only last a few rounds before separating.

: Separations are a sign of excessive headspace or as suggested above (one other cause), continual stretching usually only with very sharply tapered cases which continue to stretch. Sooner or later, the brass separates at the point of it's movement - sometimes this is the web area - it all depends on case wall construction and heat treatment. If there is no excessive stretching of the brass with frequent trimming necessary, then the cause is excessive headspace.