Headspace

[PeterPan]

Guys,
I have question to clarify something here.

I got into discussion about headspace in rimless type of cartridges like .308Win.

Here are two pictures:

where headspace is measured till the beginning of the shoulder

and

where the headspace is measured till the middle of the shoulder.

MY question is:
which is the proper way of measuring the headspace and why?

Also,
Is CIP more accurate then SAAMI?

Thanks

Peter