In the case of lap joint flanges the 'stub' is the actual portion on which the gasket sits. The stub inner diameter (and the gasket ID) is smaller than the inner diameter of the 'loose' flange.

(The ASME code still defines the flange inner diameter as the inner diameter of the loose flange.)

The gasket reaction diameter for a lapjoint flange is the midpoint between the flange and the lap (stub). So my guess would be to use the stub OD as the gasket OD, and the flange ID as the gasket ID.

Is this right?