The calculation of Ag defined in ASME B16.34 sections 6.4.1 and 6.4.2 as it pertains to ball valves.  Specifically, we are discussing section 6.4.2 and the bolts between the adapters and body.  As the spec reads "Ag = area bounded by the effective outside periphery of a gasket or O-ring or other seal effective periphery".the specifications that your referring to however I agree with you the bolts are tightened onto the gasket or 'o' ring and the area that reacts the bolt load can only be the od of the sealing face less the id of the bore.
To get the load in the bolts it would be conservative to take the system pressure and multiply the od of the seal face to get the total force and I see nothing wrong with that but when you seal the joint with the gasket to get the compressive stress in the gasket it can't be anything else but the cross sectional area of the gasket.

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