The PT rating of flanges depends for some material groups on a reference thickness vR. The definition for the vR might be unclear from the EN 1092-1 standard.
The EN 1092-1 explains the background of vR in F.2.4, however only that for different reference thicknesses there are individual PT ratings.
In Annex G.1.2 it states that 'The graduation of vR depends from the product of material from which a flange is manufactured. For the different methods of fabrication see Table 1 of this European Standard'.
In Tabel 1 you will see that a weld neck flange can be forged, machined or for > DN700 bended and electric welded.
Therefore you have to consider the manufacturing method. In case of a forged flange, the hub might be additional forged.
An important statement is in para. 5.1.2: "... flange dimensions C1 to C4 or F shall not be less than 80 % of „vR” for forgings ...". These dimensions are related to the flange ring thickness. This would lead to the assumption that vR would be the flange thickness (at bolt hole) plus the raised face (optionally divided by 0.8), if applicable.